You (4771) (su) is emphatic (placed first in the Greek sentence) clearly bringing out that Paul is making a direct appeal to Timothy. But like a good spiritual father

Paul speaking from his "longing" heart (see note 2 Timothy 1:4) adds the tender affirmation my son (teknon) for he knows that to truly motivate others, one must express genuine and unqualified concern for their full spiritual blessing and let them know that they are loved without reservation.

How are you motivating your children in the Lord...with legalism or love?

Son (5043) (teknon) (Click discussion of my beloved son in 2 Ti 1:2) refers to a child as viewed in relation to his parents or family and takes on special theological significance when the Bible calls believers the children of God. not because of our "childlikeness" but to affirm the fact that we are members of God's family and thus heirs (see note Romans 8:17) to the "family fortune". In a similar sense Timothy having been "bequeathed" the "family" treasure of the gospel was to guard it with his life. New Testament disciples are called children of their teachers, because the latter by their instruction nourish the minds of their pupils and mold their characters so that they take on "family characteristics". 

Hiebert comments that...

the tender address "my child" reminds him that what is now to be urged upon him comes from the tender, yearning heart of his spiritual father. (2 Timothy by D. Edmond Hiebert).

Therefore (3767) (oun) refers to what has gone before and introduces a logical result or inference from the preceding discourse. With that in mind what had Paul just related that dictated that Timothy needed to be strong in grace in Christ? Although chapter breaks tend to "disrupt" the flow of thought in a letter, if one observes the nearest context it is obvious that Paul had just given Timothy the reminder of the spiritual defection stating "that all who are in Asia turned away" which would surely have been disheartening to his young disciple (see note 2 Timothy 1:15). But Paul had also contrasted the sad defection with the noble example of Onesiphorus.  In addition, in a more general context Paul had made it very clear throughout chapter 1 that carrying the treasure of the gospel would bring suffering. So Paul begins this exhortational-instructional portion of the letter by emphasizing the need for personal "extrinsic" empowerment for ministry. Paul is not appealing to Timothy to "pull himself up by his bootstraps" so to speak or to "get a grip" on his emotions and courage in his own strength (see below).

Paul uses multiple pictures in this chapter to illustrate various facets of the Christian life and each one is worthy of mediation:

Son (2 Timothy 2:1)
Teacher (2 Timothy 2:2)
Soldier (2 Timothy 2:3; 2:4)
Athlete (2 Timothy 2:5)
Farmer (2 Timothy 2:6)
Prisoner (2 Timothy 2:9  2:10)
Workman (2 Timothy 2:15)
Vessel (2 Timothy 2:21)
Bondservant (2 Timothy 2:24)

BE STRONG IN (BY MEANS OF) THE GRACE THAT IS IN CHRIST JESUS: endunamou (2SPPM) en te chariti te en Christo Iesou: (1:7; Josh 1:7; Hag 2:4; 1Co 16:13; 2Pe 3:18) (See Torrey's Topic "Grace")

be strong through the grace that is ours in union with Christ Jesus (TEV)

be clothed with inward strength (Wuest)

find your strength in the grace which is in Christ Jesus (Barclay)

you must let Him make you strong (CEV )

be strong with the special favor God gives you in Christ Jesus (NLT)

be strong (strengthened inwardly) in the grace (spiritual blessing) that is [to be found only] in Christ Jesus" (AMP)

Be strong  (1743) (endunamoo from en = in + dunamóo  = strengthen) Dunamóo is derived from dúnamis which means to be able or to have power (Click for in depth word study of dunamis).

Dúnamis refers to inherent strength residing in a thing by virtue of its nature. The best spiritual example is the "gospel" which is the inherent, omnipotent power of God operating in the salvation of a lost soul that accepts it.

Dúnamis (Click for study of dunamis) means power in the sense of that which overcomes resistance or effects a change.

Dúnamis is used in the NT to speak of miracles or supernatural acts which have in them the inherent power of God or in which one sees His supernatural power exerted in their performance.

Endunamoo is used 7 times in the NT in the NASB (Acts; Ro; Eph; Phil; 1Ti; 2x 2 Ti) and is translated as: grew strong, 1; increasing in strength, 1; strengthened, 2; strengthens, 1; strong, 2.

Endunamoo in simple terms means to put power in (like a car needs gas for power) and so to make strong, vigorous, to strengthen, or to be strengthened, enabled or empowered inwardly. This word is found only in biblical and ecclesiastical Greek. The idea is to cause one to be able to function or do something. It can refer to physical strengthening as in (Heb 11:34) but more often endunamoo refers to spiritual or moral strengthening as in the case of

Abraham who "with respect to the (humanly speaking impossible) promise of God (of the birth of Isaac in his old age by Sarah), he did not waver (was not divided, did not vacillate between two opinions - belief and unbelief - implies mental struggle) in unbelief, but grew strong (endunamoo - was endued with strength or empowered) in faith (Godly faith is not full understanding but full trust), giving glory to God (see note Romans 4:20) Isaac was the result of a biological miracle performed by God in answer to Abraham’s faith.  Godly faith glorifies God; the One Who gives faith receives all the credit.

Paul is not appealing to Timothy to summon up and assert his own strength. He is not asking him to "be strong" but "be strengthened," to be made strong.

He is to let the Lord fill him with strength. Paul does not use the active but the passive voice -- he is not telling Timothy to be strong in his own strength (that would be active voice indicating the subject performs the action of strength "training" so to speak). The passive voice indicates that the source of strengthening comes from without or from a source independent of the subject.

Wuest tries to picture the idea translating it "be clothed with inward strength". Paul is saying be strengthened or be made strong. Do you see the important distinction brought out by the passive voice?

THINK ABOUT YOUR CAR...YOU DRIVE UP TO THE GAS STATION WHEN YOU ARE LOW ON FUEL....YOU PUT THE GAS IN YOUR CAR...YOUR CAR'S ENGINE IS "STRENGTHENED" BY POWER FROM A SOURCE THAT IS OUTSIDE THE CAR.

Paul issues this important directive as a command (imperative mood) to be carried out continually (present tense). Timothy is to be like a "vessel" and to allow the Lord to fill him with His power and strength. Remember that God is opposed to the proud but gives grace to the humble and that when we are weak (our strength), then and only then are we strong (His strength)! So even though endunamoo is passive voice, this experience of being made strong still involves "co-operation" of the one being strengthened. The believer is not just a passive recipient of this strength; he must actively, continually appropriate the source of strength lying in God's grace in Christ Jesus. It is not simply "Let go and let God." The believer is not simply a passive recipient of Christ's strength, but must willingly receive, continually rely and depend on the Source of strength in God's "grace which was granted us in Christ Jesus from all eternity" (see note 2 Timothy 1:9).

Remember that Paul has already told Timothy that God has...given you a spirit of...power. (see note 2 Timothy 1:77) and yet here we see that Timothy and you and I are to believe and receive the power that is already ours because of the promise of God. In other words Timothy doesn't so much need to pray for power but he needs to learn to rely and totally depend on the power that is available to him in Christ, Whose Spirit indwells him and Who is the source of that power which all based on grace. Does this make sense? If not see (note 2 Timothy 2:8) where God says if we consider what Paul writes, He will give us understanding into these deeper and yet vital truths!

The idea is similar Paul's charge to the saints at Ephesus in preparation for the rigors of spiritual warfare to be (passive voice) strong (endunamoo - present tense = continually be strengthened) in the Lord...(why?) for our struggle is not against flesh and blood... (see sermon   Ephesians 6:10,12).

Luke uses this same verb in describing Paul's conversion writing that

Saul kept increasing in strength (endunamoo - passive voice = was being empowered - progressive increase in strength as Jewish opposition grew) and confounding (this word in English implies temporary mental paralysis caused by astonishment or profound abasement) the Jews who lived at Damascus by proving ("knitting together" - continually putting together in his mind the OT clues and concluding Jesus was the Messiah) that this Jesus is the Christ. (Acts 9:22)

In his first letter to Timothy Paul (probably alluding to his empowerment in Acts) thanked

Christ Jesus our Lord, Who" had "strengthened (endunamoo)" him, because He considered (him) faithful, putting (him) into service. (1Ti 1:12)

God’s sovereign purpose for Paul and for all believers works through personal faith. Until Paul was turned by the Holy Spirit from self-righteous works to faith alone in Christ, he could not be empowered and used by God.

In his last words to Timothy, Paul illustrated his personal example of being strong in the grace that is in Christ Jesus writing that in the face of everyone deserting him (as might happen to Timothy or any disciple who retains the standard of sound words),

"the Lord stood with me (fulfilling His promise never to “leave or forsake” His children) and strengthened (endunamoo) me, in order that through me the proclamation might be fully accomplished, and that all the Gentiles might hear; and I was delivered out of the lion's mouth." (see note 2 Timothy 4:17)

Paul is saying to Timothy in essence that "in your weakness He will be your strength".  It is worth noting that endunamoo is used in (Acts 9:22) at the beginning of Paul's public ministry, where we noted above that he "kept increasing in strength" and then is used here in 2 Timothy 4:17 which marks the end of his public ministry—a poignant picture of this Paul's continual dependence upon the sustaining strength of the Lord!

Paul reminds the saints at Philippi that he had learned the secret of contentment (see note Philippians 4:11-12)

"I can do all things through Him Who strengthens (endunamoo - continually strengthens)" him (see note Philippians 4:13).

The Amplified Version translates this verse in Philippians... 

I am ready for anything and equal to anything through Him Who infuses inner strength into me; I am self-sufficient in Christ’s sufficiency. 

John MacArthur notes commenting on this passage in Philippians adds that Paul does not...

mean that he could physically survive indefinitely without food, water, sleep, or shelter. What he is saying is that when he reached the limit of his resources and strength, even to the point of death, he was infused with the strength of Christ. He could overcome the most dire physical difficulties because of the inner, spiritual strength God had given him. (MacArthur, J. Philippians. Chicago: Moody Press)

Dear reader. Are you weak today? in distress? insulted? in difficulty? persecuted?. Then consider imitating Paul and "boast" about it for you are on the pathway of empowerment, for He has said

"My grace is sufficient for you, for power (dunamis) is perfected in weakness."

To which Paul responded

Most gladly, therefore, I will rather boast about my weaknesses, that the power (dunamis) of Christ may dwell in me...for when I am weak, then I am strong. (2Cor 12:9,10)

Let us therefore humbly submit to and thankfully accept (this is very difficult for me and a lesson I have to continually "re-learn") the circumstances and/or people God sends into our life as the vessels He will use to cause us to continually rely on His grace and His power (dunamis).

Grace (5485) (charis) (Click for in depth word study of charis)

W E Vine comments on "grace" as used in this context stating that

It is not here grace to the guilty but the gracious favor of God in its enabling power and effect, which is to be found alone in Christ...Only as we live in the enjoyment of the power of this grace can we devotedly and faithfully discharge the service committed to us. There is adequate grace to meet our every need. (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson ) (Bolding added)

In the grace that is in Christ Jesus indicates that the grace is the inward source of a believer's strength. Thus in is probably instrumental and could be translated by means of or in the power of His grace. 

Williams paraphrases it this way

"in the spiritual blessing that comes through union with" Christ Jesus.

The English Version (TEV) paraphrase is similar and emphasizes the Source of this grace is

in union with Christ Jesus

Our Source of strength is Christ Jesus,

the Word...full of grace and truth...of His fulness we have all received and grace upon grace...grace and truth were realized through Jesus Christ. (Jn 1:14, 1:16, 1:17).

All believers need to be mindful of our spiritual resources and resist the tendency (temptation) to rely on our own strength or otherwise we will experience discouragement, weariness, and disillusionment because spiritual warfare is a supernatural battle, which is hard and which will not stop until we are glorified. 

So in this chapter, Paul is reminding Timothy of the Source of His strength and the necessity to rely on the Source rather than self, a message frequently recorded in Scripture (Joshua 1:6-7, 1:7, 1Chr 22:13, 1Cor 16:13, see sermon Ephesians 6:10).

Spurgeon comments on "grace...in Christ" --

Christ has grace without measure in Himself, but He hath not retained it for Himself. As the reservoir empties itself into the pipes, so hath Christ emptied out His grace for His people. "Of His fulness have all we received, and grace for grace." (Jn 1:16) He seems only to have in order to dispense to us. He stands like the fountain, always flowing, but only running in order to supply the empty pitchers and the thirsty lips which draw nigh unto it. Like a tree, He bears sweet fruit, not to hang on boughs, but to be gathered by those who need. Grace, whether its work be to pardon, to cleanse, to preserve, to strengthen, to enlighten, to quicken, or to restore, is ever to be had from Him freely and without price; nor is there one form of the work of grace which He has not bestowed upon His people. As the blood of the body, though flowing from the heart, belongs equally to every member, so the influences of grace are the inheritance of every saint united to the Lamb; and herein there is a sweet communion between Christ and his Church, inasmuch as they both receive the same grace. Christ is the head upon which the oil is first poured; but the same oil runs to the very skirts of the garments, so that the meanest saint has an unction of the same costly moisture as that which fell upon the head. This is true communion when the sap of grace flows from the stem to the branch, and when it is perceived that the stem itself is sustained by the very nourishment which feeds the branch. As we day by day receive grace from Jesus, and more constantly recognize it as coming from Him, we shall behold Him in communion with us, and enjoy the felicity of communion with Him. Let us make daily use of our riches, and ever repair to Him as to our own Lord in covenant, taking from him the supply of all we need with as much boldness as men take money from their own purse. (Spurgeon, C H: Morning and Evening) (Bolding added)

J Vernon McGee adds this pithy comment

I love this—be strong in grace. My friend, if you think that you can grit your teeth and go out and live the Christian life on your own, you’re in for a great disappointment. If you feel that you can follow a few little rules or some clever gimmicks to make you a mature Christian, then you have fallen into a subtle trap of legalism. Paul gives no rules, and the Word of God has no rules to tell the child of God how to live the Christian life. We are saved by grace, and now we are to live by the grace of God and be strong in that grace....When I hear Christians say, “I don’t do this, and I don’t do that, and I am following a set of rules,” I immediately recognize that they know very little about the grace of God. They are trying to live the Christian life in their own strength. Paul says, “Be strong in the grace that is in Christ Jesus. (McGee, J V: Thru the Bible Commentary:  Nashville: Thomas Nelson) (Bolding added)

Grace permeates this short epistle --

Paul opened with a benediction for grace (see note 2 Timothy 1:2) reminded Timothy that he was saved by grace (see note 2 Timothy 1:9) and closed with his final written words of blessing - "grace be with you".  (see note 2 Timothy 4:22).

How would Timothy "succeed" in ministry? How could he possibly be able to suffer hardship as a good soldier? Would he succeed because he studied enough, prayed enough, taught enough, endured enough, etc?

No, there was only one way to fulfill his ministry. The way he began...by the grace that is in Christ. This grace is embodied in Christ Jesus Who imparts it to all who are in union with Him, just as the vine imparts its life and fruitfulness to the branches that are abiding in union with it, for as Jesus said "apart for Me (and His "sap" of empowering grace) you can do nothing" (Jn 15:5). As believers we exist in the sphere of grace through our union with Christ. It's the grace of God that empowers believers. It's God's grace that would enable Timothy and us to please and serve God--otherwise we could do nothing of eternal value.

Peter prayed that

grace and peace be multiplied" to his readers (who were believers) "in the knowledge (epignosis = full, complete knowledge) of God and of Jesus our Lord (See note 2Peter 1:2).

From Peter's prayer for his readers, one can deduce that the deeper and wider the channel through which knowledge of the Lord flows, the more grace and peace will be multiplied, assuming that this "head knowledge" becomes "heart knowledge", leading to obedience motivated by love not legalism. The more we study and obey the "gospel of the grace of God" (Acts 20:24), the more grace shall we derive from the "Word of His grace" (Acts 14:3).

John Piper adds that

knowing God is the means by which his grace and peace become large and powerful in our lives. If you want to enjoy God's peace and be the aroma of his grace in the world, your knowledge of Him has to grow. Grace is not a mere deposit. It is a power that leads to godliness (see notes Titus 2:11, Titus 2:12) and eternal life (cf sermon note   Eph 2:8-9). And where knowledge of the glory and excellence of God languishes, grace does not flow. The channel from God's infinite reservoir of grace into and through our lives is knowledge of God. We do not study the Scripture for its own sake, but because through it comes the knowledge of God and through that, grace and peace are multiplied in your heart..." But knowledge alone will not open the floodgates of grace. We must obey what we know. To say it another way: Because we belong to Christ, we are continually in the sphere of grace (grace in which we stand - see note Romans 5:2). But to enjoy the sphere of blessing, we must live in the sphere of obedience. Peter ends his second epistle with a command to "grow in the grace and knowledge of our Lord and Savior Jesus Christ" (see note 2 Peter 3:18). Spurgeon adds that "an increase of love to Jesus and a more perfect apprehension of His love to us is one of the best tests of growth in grace. (Bolding added)

In Faith's Checkbook Spurgeon comments on the truth that

"The LORD gives grace and glory" (Ps 84:11): "Grace is what we need just now, and it is to be had freely. What can be freer than a gift? Today we shall receive sustaining, strengthening, sanctifying, satisfying grace. He has given daily grace until now, and as for the future, that grace is still sufficient. If we have but little grace the fault most lie in ourselves; for the LORD is not straitened, neither is He slow to bestow it in abundance. We may ask for as much as we will and never fear a refusal. He giveth liberally and upbraideth not. The LORD may not give gold, but He will give grace: He may not give gain, but He will give grace. He will certainly send us trial, but He will give grace in proportion thereto. We may be called to labor and to suffer, but with the call there will come all the grace required; What an "end" is that in the text -- "and glory!" We do not need glory yet, and we are not yet fit for it; but we shall have it in due order. After we have eaten the bread of grace, we shall drink the wine of glory. We must go through the holy, which is grace, to the holiest of all, which is glory. These words and glory are enough to make a man dance for joy. A little while -- a little while, and then glory forever!" (Bolding added)

The way upward to the throne of grace to receive grace in the nick of time is downward in our estimation of our abilities and our strength. Spurgeon (Faith's Checkbook) commenting on the fact that "God is opposed to the proud but gives grace to the humble" (Js 4:6, see note 1 Peter 5:5, Pr 3:34) that

Humble hearts seek grace, and therefore they get it. Humble hearts yield to the sweet influences of grace, and so it is bestowed on them more and more largely. Humble hearts lie in the valleys where streams of grace are flowing, and hence they drink of them, Humble hearts are grateful for grace and give the LORD the glory of it, and hence it is consistent with His honor to give it to them. Come, dear reader, take a lowly place. Be little in thine own esteem, that the LORD may make much of thee. Perhaps the sigh breaks out, "I fear I am not humble." It may be that this is the language of true humility. Some are proud of being humble, and this is one of the very worst sorts of pride. We are needy, helpless, undeserving, hell-deserving creatures, and if we are not humble we ought to be. Let us humble ourselves because of our sins against humility, and then the LORD will give us to taste of His favor. It is grace which makes us humble, and grace which finds in this humility an opportunity for pouring in more grace. Let us go down that we may rise. Let us be poor in spirit that God may make us rich. Let us be humble that we may not need to be humbled but may be exalted by the grace of God." Spurgeon adds that "Our troubles have always brought us blessings, and they always will. They are the black chariots of bright grace." "Our LORD will give us grace to follow the most difficult paths of duty without a stumble. He can fit our foot for the crags so that we shall be at home where apart from God we should perish." "It seems that Jehovah's way is to lower those whom He means to raise and to strip those whom He intends to clothe. If it is His way, it is the wisest and best way. If I am now enduring the bringing low, I may well rejoice, because I see in it the preface to the lifting up. The more we are humbled by grace, the more we shall be exalted in glory. That impoverishment which will be overruled for our enrichment is to be welcomed. (Bolding added)

Commenting on "My grace is sufficient" (2Cor 12:9,10) Spurgeon writes

Our weakness should be prized as making room for divine strength. We might never have known the power of grace if we had not felt the weakness of nature. Blessed be the LORD for the thorn in the flesh, and the messenger of Satan, when they drive us to the strength of God. This is a precious word from our LORD's own lip. It has made the writer laugh for joy. God's grace enough for me! I should think it is. Is not the sky enough for the bird and the ocean enough for the fish? The All-Sufficient is sufficient for my largest want. He who is sufficient for earth and heaven is certainly able to meet the case of one poor worm like me. Let us, then, fall back upon our God and His grace. If He does not remove our grief, He will enable us to bear it. His strength shall be poured into us till the worm shall thresh the mountains, and a nothing shall be victor over all the high and mighty ones. It is better for us to have God's strength than our own; for if we were a thousand times as strong as we are, it would amount to nothing in the face of the enemy; and if we could be weaker than we are, which is scarcely possible, yet we could do all things through Christ. (Bolding added)

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What Comes Naturally? - The story is told about an elderly man who retired after many years in the British Army. One day a man who knew about his long and distinguished military career decided to play a prank on him. As the old soldier walked down the street with his arms full of packages, the jokester sneaked up behind him and shouted, "Attention!" Without hesitation, the military man dropped his arms to his side, and every package went tumbling to the sidewalk. Without a conscious thought, the veteran was doing what comes naturally for a soldier.

Similarly, as believers in Christ, we should respond in a manner that corresponds with our new life. Our behavior is to be more and more in line with the example of Jesus' life. We still must deal with sinful desires, so we need to discipline ourselves to be the kind of person God wants us to be. Like a soldier or athlete in training (2 Timothy 2:3-5), we need to practice repeatedly until doing what's right comes naturally.

Through faith in Christ we are children of the heavenly Father. By the power of the indwelling Spirit, therefore, let us develop the habit of submitting to God's Word. Then, in every situation of life we will increasingly find that obeying Him is "doing what comes naturally." — Richard De Haan (Our Daily Bread, Copyright RBC Ministries, Grand Rapids, MI. Reprinted by permission. All rights reserved)

Lord, may our lips and lives express
The blessed gospel we profess;
So let our works and virtues shine
And speak of Him who is divine. —Anon.

When we walk with Christ,
we become more like Him.

I love Annie Johnson Flint's poem on grace below (click here for more of her poetry)