Note that God (theos) is placed first in the Greek to emphasize His vital role in this process.

Wuest comments that...

In verse twelve, we have human responsibility, in verse thirteen, divine enablement, a perfect balance which must be kept if the Christian life is to be lived at its best. It is not a “let go and let God” affair. It is a “take hold with God” business. It is a mutual co-operation with the Holy Spirit in an interest and an activity in the things of God. The saint must not merely rest in the Holy Spirit for victory over sin and the production of a holy life. He must in addition to this dependence upon the Spirit, say a positive NO to sin and exert himself to the doing of the right (cp the teaching, child rearing role of the "grace of God" in Titus 2:12-see notes). Here we have that incomprehensible and mysterious interaction between the free will of man and the sovereign grace of God. (Wuest, K. S. Wuest's Word Studies from the Greek New Testament: Eerdmans or Logos) (Bolding added)

CEV paraphrases it...

God is working in you to make you willing and able to obey him.

God calls us to holiness, and then empowers us to pursue holiness.

For (1063) (gar) or "because" introduces an explanation and in context explains how it is possible for believers to obey the command to continually work out their salvation. This verse explains God's role (God's sovereignty) in the believer's sanctification process whereas the preceding verse explains our role (man's responsibility). It should be clear that without God "working in" the believer who is "working out" his or her own salvation, genuine sanctification would be impossible.

It is notable that the teaching that they are enabled to obey by God’s power is virtually unparalleled in pre-Christian literature except for Old Testament teachings on the Holy Spirit.

Paul places God (2316) (theos) first in the Greek sentence, which emphasizes the critical role God plays in our ability to work out our own salvation. God gives us both the desire and the energy. God's Holy Spirit, the Spirit of Christ lives in each believer and He gives us the desire and the energy to

"not walk according to the flesh, but according to the Spirit"...and enables us "by the Spirit...(to put) to death the deeds of the body." In short we are to be continually "led by the Spirit" of the Living God Who is continually at work in us and Who Alone "is able to keep (us) from stumbling, and to make (us) stand in the presence of His glory blameless with great joy" (Romans 8:4-note, 8:13-note, Galatians 5:18-note Jude 24)

In the Old Testament we see God at work in Judah...

The hand of God was also on Judah to give them one heart to do what the king and the princes commanded by the word of the LORD. (2Chr 30:12)

Isaiah records during the Millennium that the Jews (all of whom will be redeemed at that time) will acknowledge....

LORD, you will grant us peace, for all we have accomplished is really from you. (Isa 26:12, NLT)

F F Bruce writes that...

When the Spirit takes the initiative in imparting to believers the desire and the power to do the will of God, then that desire and power becomes theirs by His gift, and they do His will ‘from your heart’ (See note Ephesians 6:6)

As the apostle Peter declared...

His divine power has granted (perfect tense = speaks of the permanence of this grant) to us everything pertaining to life and godliness, through the true knowledge of Him who called us by His own glory and excellence. (See note 2 Peter 1:3)

Work (1754) (energeo from energes = active, operative, at work in turn from en = in + érgon = work) refers to active, efficient, effectual fervent work. God energizes His children to obey and serve Him; His power enables their sanctification.

Paul describes God's effective energetic power in believers, alluding to the operation of the Holy Spirit and the transforming power of grace. The present tense indicates God is continually at work energizing believers, enabling them to work out their salvation. Don't be discouraged beloved, for Paul is saying God is always at work in us. It is for that reason that sanctification will continue throughout the believer’s life (see note Philippians 1:6). Those whom God justifies by grace through faith He just as surely sanctifies (also by grace through faith).

If you are discouraged by failures, the truth that God is continually at work in you and clearly has not given up on you should encourage you to forget what lies behind and press on in His power knowing that it is always too soon to quit!

Paul did not underestimate the importance of faithful obedience, but he knew that underlying all our obedience and acceptable service was the energizing power and will of God, Who Alone then will receive the glory. It is as if believers who are working out their salvation are God's "trophies" before the lost, watching world! Beloved, is your "trophy" shining forth or do you need to "dust" it off by practicing the principles of Philippians 2:12-13?

Paul emphasized this same principle of God's inner working and thus our dependence on God's power writing to the Corinthians...

Not that we are adequate in ourselves to consider anything as coming from ourselves, but our adequacy is from God, who also made us adequate as servants of a new covenant, not of the letter, but of the Spirit; for the letter kills, but the Spirit gives life. (2Corinthians 3:5-6)

After declaring that his great desire and purpose was to present all men complete in Christ (see note Colossians 1:28), he went on to explain how he carried out this task writing that it was...

for this purpose also I labor (kopiao to the point of exhaustion in the present tense = continually laboring), striving (agonizomai intensely struggling like an athlete in the present tense = continually striving) (Paul's responsibility) according to His power (God's provision), which mightily (dunamis) works (energeo in the present tense = continually energizes) within me . (See note Colossians 1:29) (Paul was passionate to see men formed complete in Christ and we should be no less zealous.)

In his letter to the Ephesians Paul emphasized that the carrying out of his responsibility was made possible by God's empowerment...

(Paul  reminded them that he) was made a minister, according to the gift of God's grace which was given to me according to the working (energeia in this context = supernatural energy) of His power (dunamis). (see note Ephesians 3:7)

Now to Him Who is able (dunamai in the present tense = continually has the inherent ability - see omnipotence) to do exceeding abundantly beyond all that we ask or think, according to the power (dunamis - Inherent power residing in a thing by virtue of its nature - obviously God's supernatural power) that works (energeo in the present tense = continually energizes) within us, to Him be the glory in the church and in Christ Jesus to all generations forever and ever. Amen” (see notes Ephesians 3:20; 3:21).

Paul's point is that God energizes His children to obey and serve Him! His energy enables our ongoing, daily supernatural process of sanctification. In fact, believers can do nothing holy or righteous in their own power or resources and this even includes "church work" (especially if that work is done in our own natural [rather than supernatural] power and for our "recognition"!) (cp Jesus' warning that "apart from Me you can do nothing." John 15:5)

God is the Energy
and
The Energizer

William Hendriksen explains the working out process with several analogies writing that...

The toaster cannot produce toast unless it is “connected,” so that its nichrome wire is heated by the electricity from the electric power house. The electric iron is useless unless the plug of the iron has been pushed into the wall outlet. There will be no light in the room at night unless electricity flows through the tungsten wire within the light-bulb, each end of this wire being in contact with wires coming from the source of electric energy. The garden-rose cannot gladden human hearts with its beauty and fragrance unless it derives its strength from the sun. Best of all, “As the branch cannot bear fruit of itself unless it abides in the vine, so neither can you unless you abide in me” (John 15:4).So here also. Only then can and do the Philippians work out their own salvation when they remain in living contact with their God...By means of his Spirit working in the hearts of his people (see note Philippians 1:19), applying to these hearts the means of grace and all the experiences of life, God is the great and constant, the effective Worker, the Energizer, operating in the lives of the Philippians, bringing about in them both to will and to work. Note: not only to work but even to will, that is, to resolve and desire. (Hendriksen, W., & Kistemaker, S. J. Vol. 5: New Testament commentary : Exposition of Philippians. Page 122. Grand Rapids: Baker Book House or Logos)

BOTH TO WILL AND TO WORK: kai to thelein (PAN) kai to energein (PAN) : (1Ki 8:58; 1 Chr 29:14-18; Ezra 1:1,5; 7:27; Neh 2:4; Ps 110:3; 119:36; Ps 141:4; Pr 21:1; Jn 6:45,65; Ep 2:4,5; 2Th 2:13,14; Titus 3:4,5; 1Pet 1:3)

God produces the desire to live godly and provides the effective energy to accomplish this supernatural objective in the life of every believer. So what is your excuse?

As Wiersbe rightly remarks...

Too many Christians obey God only because of pressure on the outside, and not power on the inside.

Are you wrestling with what is the Will of God for you life? You might want to read the RBC booklet How Can I Know What God Wants Me To Do?

In Philippians 2:12 and 2:13, Paul has in view both human choice (responsibility) and God’s sovereignty (provision/power). When Spurgeon was asked to “reconcile” the two, he replied,

How do I reconcile friends?

Will (2309) (thelo) means to determine and refers to one's desire and implies volition and purpose. Thelo refers to thoughtful, purposeful choice, not to mere whim or emotional desire. A genuine desire to do God’s will, as well as the power to obey it, originates with Him.

First volition (will)
Then action (work)

Believers choose to behave a certain way but only because the Holy Spirit is at work causing us to want to do God’s will. God arouses, stirs, and energizes the heart of the believer to do God's will. This is a wonderful truth. All believers experience movements and stirrings within their heart toward God. These stirrings are from His Spirit. God is working within —energizing —giving both the will and power to do what pleases Him. Amazing grace! Our part is to lay hold of these stirrings and not to let them pass by unheeded. We are to grab hold of them and do exactly what the stirrings are arousing and energizing us to do. Then we are truly working out our salvation. Praise God He does not leave us to our own futile efforts.

In ancient secular Greek thelo was used by Homer to speak of readiness, inclination, and desire. When one was ready for an event, or inclined to undertake a course of action, thelo was used. In the writings of Plato the word came to speak of intention or desire.

And so we learn that God’s work in us includes the transformation of our will, as well as our work. But clearly His work is not a passive transaction, in light of the exhortation in the preceding verse to work out our own salvation.

John MacArthur has an interesting comment on this passage writing that...

God uses two means to move believers’ wills. First is what might be called holy discontent, the humble recognition that one’s life always falls short of God’s standard of holiness...

The second means God uses to move believers’ wills is holy aspiration, the positive side of holy discontent. After He instills a genuine hatred of sin, He cultivates a genuine desire for righteousness. After He makes believers discontent with what they are, He gives them the aspiration to greater holiness. Above all, it is the desire to be like Christ, “to become conformed to the image of [God’s] Son” (see note Romans 8:29)...

Holy resolve leads to holy living. A godly will produces godly work. (Read the full message on "God At Work in You" Part 3)

And to work - The power that works in us and "energizes" our new supernatural life, is the power of the Holy Spirit of God (cp John 14:16-17, 26; Acts 1:8; 1 Cor. 6:19-20). We do well to remember that the same Holy Spirit Who empowered Christ when He was ministering on earth is to empower us as well. Luke describes the Holy Spirit's empowering role in Jesus' life and ministry...

And Jesus, full of the Holy Spirit, returned from the Jordan and was led about by the Spirit in the wilderness (Luke 4:1)

And Jesus returned to Galilee in the power (dunamis) of the Spirit; and news about Him spread through all the surrounding district. 15 And He began teaching (What was His source of "power" with which to teach?) in their synagogues and was praised by all. (Luke 4:14-15)

You know of Jesus of Nazareth, how God anointed Him with the Holy Spirit and with power (dunamis), and how He went about doing good, and healing all who were oppressed by the devil; for God was with Him. (Acts 10:38)

Jesus promised the same Spirit and power to His disciples and the Spirit is still every believer's source of power...

(Jesus said) you shall receive power (dunamis) when the Holy Spirit has come upon you; and you shall be My witnesses both in Jerusalem, and in all Judea and Samaria, and even to the remotest part of the earth."

Work (1754) (energeo from energes = active, operative, at work in turn from en = in + érgon = work) refers to active, efficient, effectual fervent work. It refers to being energized and active in a particular endeavor.

God energizes His children to obey and serve Him; His power enables their sanctification. Energeo in the NT virtually always describes supernatural activity, principally God's energizing activity and this verse is no exception.

Energeo describes active, efficient, effective working. Paul is saying that God exerts effective, energetic power in believers which enables them to obey. The activity put forth in an individual energizes him to the doing certain things intended by God Who is doing the energizing.

The present tense indicates that God continually works effectually and productively, providing the necessary power for supernatural living.

Paul linked this divine internal working or energizing in believers with the living and abiding Word of God writing to the saints at Thessalonica...

And for this reason we also constantly thank God that when you received from us the word of God's message, you accepted it not as the word of men, but for what it really is, the word of God, which also performs (energeo = effective, operative and productive, continually [present tense] producing an effect in the lives of those who receive it) its work in you who believe.  (See note 1Thessalonians 2:13)

The prayer of the writer of Hebrews echoes a similar dependence on God's power to carry out what He calls us to do, the writer asking that God...

equip you in every good thing to do His will, working (present tense = continually) in us that which is pleasing (euarestos = well pleasing, acceptable, speaks of God's attitude toward man) in His sight, through Jesus Christ, to whom be the glory forever and ever. Amen. (see note Hebrews 13:21)

A T Robertson writes...

 “Both the willing and the working (the energizing).” God does it all, then. Yes, but he puts us to work also and our part is essential, as he has shown in verse 12, though secondary to that of God.

William Barclay commenting on the meaning of energeo notes that...

There are two significant things about (energeo); it is always used of the action of God, and it is always used of effective action. God’s action cannot be frustrated, nor can it remain half-finished; it must be fully effective. (Barclay, W: The Daily Study Bible Series, Rev. ed. Philadelphia: The Westminster Press)

A BALANCED VIEW

Warren Wiersbe explains this balance writing that Paul...

is setting before us the divine pattern for the submissive mind and the divine power to accomplish what God has commanded. “It is God which worketh in you” (Phil. 2:13). It is not by imitation, but by incarnation—“Christ liveth in me” (Gal. 2:20). The Christian life is not a series of ups and downs. It is rather a process of “ins and outs.” God works in, and we work out. We cultivate the submissive mind by responding to the divine provisions God makes available to us. (Wiersbe, W: Bible Exposition Commentary. 1989. Victor)

Greg Herrick reminds believes of the need to "keep our balance" in our Christian walk:

We cannot say, “It all depends on me. This makes Christianity just a list of do’s and don’ts.” This negates verse 13. Yet, on the other hand, we cannot sit around waiting for God to do something, all the while disobeying the explicit teaching of Scripture. This is to deny the imperative in verse 12. The informed Christian who knows the Lord through his word, and in prayer, will say with the apostle Paul:

by the grace of God I am what I am, and His grace toward me did not prove vain; but I labored even more than all of them, yet not I, but the grace of God with me. (1 Corinthians 15:10).

Jeremy Taylor wrote that...

God has given to man but a short time on earth, yet upon this time does all eternity depend.

Henry Drummond writes that...

One of the futile methods of sanctifying ourselves is trying; effort--struggle--agonizing. I suppose you have all tried that, and I appeal to your own life when I ask if it has not failed. Crossing the Atlantic, the Etruria, in which I was sailing, suddenly stopped in mid-ocean--something had broken down. There were a thousand people on board that ship. Do you think we could have made it go if we had all gathered together and pushed against the sides or against the masts? When a man hopes to sanctify himself by trying, he is like a man trying to make the boat go that carries him by pushing it--he is like a man drowning in the water and trying to save himself by pulling the hair of his own head. It is impossible. Christ held up the mode of sanctification almost to ridicule when He said: "Which of you by taking thought can add a cubit to his stature?" Put down that method forever as futile.

Another man says: "That is not my way. I have given up that. Trying has its place, but that is not where it comes in. My method is to concentrate on some single sin, and to work away upon that until I have got rid of it." Now, in the first place, life is too short for that process to succeed. Their name is legion. In the second place, that leaves the rest of the nature for a long time untouched. In the third place, it does not touch the seed or root of the disease. If you dam up a stream at one place, it will simply overflow higher up. And for a fourth reason: Religion does not consist in negatives--in stopping this sin and stopping that sin. (Henry Drummond)

John Piper reconciles Philippians 2:12 and 2:13 this way...

God's sovereignty in sanctification does not remove our obligation. It enables it...God's sovereign work in us is our only hope that we will press on to maturity. (from Let Us Press on to Maturity) God’s working and willing in us does not make our working pointless; it makes it possible. (from Assessing Ourselves ) We obey and we work. It is our act and our choice. But beneath our doing and our willing is God giving the willing and giving the doing. "For it is God who is at work in you, both to will and to work for his good pleasure." It is really our work and really his gift. It is really our willing and really his gift. (from Let Us Press On To Maturity Hebrews 6:1-3)

Lehman Strauss writes that...

 We work and God works. It is a mutual effort toward the common goal of glorifying God in our lives. Here is a blending and interacting of God’s sovereign grace and power and man’s free will. God works in us but we dare not be passive. We work, too, and our work and the exercise of our wills are never at greater liberty than when thus engaged in doing ‘His good pleasure.’ The Holy Spirit abides in the believer, and he is never more pleased than when we are working out that which He has worked in...But remember, while God has assumed the responsibility for the inworking, we are responsible for the outworking” (Studies in Philippians, p. 123).  (Bolding added)

As C H Spurgeon put it...

We must work out our own salvation with fear and trembling, but not till he has worked in us can we work it out.

.In a similar reminder Oswald Chambers writes that...

God alters our disposition, but he does not make our character. When God alters my disposition, the first thing the new disposition will do is to stir up my brain to think along God’s line. As I begin to think, begin to work out what God has worked in, it will become character. Character is consolidated thought. God makes me pure in heart; I must make myself pure in conduct.

C S Lewis commented that...

Scripture just sails over the problem [of the whole puzzle about grace and free will]. “Work out your own salvation in fear and trembling” – pure Pelagianism. But why? “For it is God who worketh in you” – pure Augustinianism (he argued that without grace there could be no salvation). It is presumably only our presuppositions that make this appear nonsensical.

Chuck Swindoll in his exposition of Philippians (Laugh Again) writes that...

Christ says in effect, “You want to live My life? Here is My power.” Lo and behold, He strengthens us within. “You want to please My heavenly Father? Here’s My enablement.” And He enables us by His Spirit...You see, Christ not only lived an exemplary life, He also makes it possible for us to do the same. He gives us His pattern to follow without, while at the same time providing the needed power within...Because we have His example to follow and His power to pull it off, you and I no longer have to fake it or hurry it or strive for it. Once He gets control of our minds, the right attitudes bring about the right actions (Laugh Again, p. 96 ).

F B Meyer writes...

He works in us to will. That is, He does not treat us like a machine. He deals with us as moral agents who can say yes and no. He is not going to compel us to be saints, He is not going to force us to be holy. If thou wilt, He much more wills, and thou dost will because He willed before. The will of God wants to take thee up into itself, as the wind that breathes over a city waits to catch up the smoke from a thousand chimney-pots, and waft it on its bosom through the heavens.

You may always know when God is willing within you--

First, by a holy discontent with yourself. You are dissatisfied with all that you have ever done, and been.

Secondly, you aspire; you see above you the snow-capped peaks, and your heart longs to climb and to stand there.

Thirdly, these are followed by the appreciation of the possibility of your being blameless and harmless and without rebuke. If a man refuses to believe that he can be a saint, he never will become one. If a man says, I cannot hope to be more than conqueror, God Himself cannot save him. When the Spirit of God is within you, there rises up a consciousness that you have the capacity for the highest possible attainments, because you were made and redeemed in the image of God, and because the germ of the Christ-nature has been sown in your spirit. Two men go through a picture-gallery. Each sees the same masterpiece. One says, I cannot imagine how that can be done. The other man says, I also am a painter. That second man is capable of producing a picture which also shall outlive. You must believe that you can be a saint, even you. You must dare to believe it, because the Christ-germ is sown in your character, and because God is working in you to will and to do.

Fourthly, the determination, I will. There should be a moment in the history of us all when each shall say--Cost what it may, I will not yield again; I will arise to be what God wants to make me; I will yield myself to Him; I will reckon myself to be dead indeed unto sin, and alive unto God through Jesus Christ; I will yield myself to the power that worketh in me. Discontent, aspiration, appreciation of the possibilities of saintliness, and resolve.

The will of God is working in you to-day. Cannot you take those four steps? Are you going back to live the old self-indulgent life? If so, these words will be a curse to you, for nothing injures the soul so much as to know the truth and yet fall back into the ditch. (Devotional Commentary on Philippians)

FOR HIS GOOD PLEASURE: huper tes eudokias: (Lu 12:32; Ro 9:11,16; Eph 1:5,9,11; 2:8; 2Th 1:11; 2Ti 1:9)

The NIV is slightly different rendering Philippians 2:13...

for it is God who works in you to will and to act according to his good purpose.

The New Living paraphrase renders it...

For God is working in you, giving you the desire to obey him and the power to do what pleases him.

Clarke writes that...

Every good is freely given of God; no man deserves any thing from Him; and as it pleases Him, so He deals out to men those measures of mental and corporeal energy which He sees to be necessary; giving to some more, to others less, but to all what is sufficient for their salvation.

Barnes writes that...

Here eudokia means that which would be agreeable to him; and the idea is, that he exerts such an influence as to lead men to will and to do that which is in accordance with his will.

Boice has some interesting thoughts on this passage writing...

I wonder if you have ever noticed that the well-known verses of Ephesians 2:8-10 speak twice of our works, the things that we do. One kind of work is condemned because it comes out of ourselves and is contaminated by sin. The other kind of work is encouraged because it comes from God as he works within the Christian. The verses say,

"For it is by grace you have been saved, through faith—and this not from yourselves, it is the gift of God—not by works [that is, of human working], so that no one can boast. For we are God's workmanship, created in Christ Jesus to do good works [that is, the result of God's working], which God prepared in advance for us to do."

These verses are really Paul's own commentary upon Philippians 2:12-13, for they tell us that although God can never be satisfied with any good that comes out of human beings, he is satisfied and pleased with the good that is done by Christians through the power of Jesus Christ within them. Through that power the tyranny of sin is broken, the possibility of choosing for God is restored, and a new life of communion with God and holiness is set before the Christian. (Boice Expositional Commentary)

Good pleasure (2107) (eudokía from eu = well, well off + dokeo = to seem, to think, to have an opinion)  means good will or pleasure. Eudokia speak of that which pleases.

Please note, eudokia (in my opinion) is one of those Greek words which is somewhat difficult to define in concrete, easily apprehended terms, so keep this caveat in mind as you read the various definitions of eudokia. Part of the difficulty in defining eudokia arises from the fact that it has no classic Greek uses, appearing for the first time in Septuagint.

In Romans 10:1 eudokia describes a feeling of strong emotion in favor of and thus a desire or wish and includes the idea that a desire is usually directed toward something that causes satisfaction or favor. Thayer offers for this instance of its use, “desire, for delight in any absent thing easily begets a longing for it.”

 In Philippians 1:15 eudokia speaks of men and describes having a good intent or goodwill (contrasting with envy and strife). Most of the other NT uses of eudokia (including here in Philippians 2:13) are  used of God. Eudokia expresses not merely a benevolent attitude but an active pleasure, and, when used of something not yet realized, indicates a fervent desire.

God's motive behind His work in our lives is because it gives Him pleasure!