SO THIS I SAY, AND AFFIRM TOGETHER WITH THE LORD: Touto oun lego (1SPAI) kai marturomai (1SPMI) en kurio:  (1 Corinthians 1:12; 15:50; 2 Corinthians 9:6; Galatians 3:17; Colossians 2:4)  (Nehemiah 9:29,30; 13:15; Jeremiah 42:19; Acts 2:40; 18:5; 20:21; Galatians 5:3; 1 Thessalonians 4:6)

Paul is saying in this section...

"you formerly walked according to the course of this world" (2:2)

" walk no longer just as the Gentiles also walk" (4:17)

As Ruth Paxson says...

To be in Christ and not grow up into Christ makes the Christian life an anachronism, a monstrosity, a lie. The revelation of Christ in truth must result in the realization of Christ in life. Paul writes, Ye have heard and accepted "the truth as it is in Jesus"; now you must live, act and speak according to this new standard. There can be no compromising alliances, no stultifying reserves, no divided interests.

"Ye were" — "Ye are" — "Be ye"

Here is Paul's forceful challenge to become what you are. It leads very naturally into his next practical exhortation: Call to Put Off The Old and to Put On The New. (Paxson, Ruth: The Wealth, the Walk and the Warfare of the Christian. 1939. Revell)

So (3767) (oun) means therefore or consequently and is a term of conclusion. We cannot escape the word therefore (so) in this letter for Paul utilizes it serves as a sign post to arrest and compel our attention. Based upon what Paul has just explained about the importance of unity and diversity, Paul will give his readers a charge which further explains what it means to walk in a manner worthy of the calling to which we have been called. One could also see this "therefore" as a resumption of the first three verses calling for a worthy walk (see notes Ephesians 4:1; 4:2; 4:3) Here Paul begins his  appeal for a new morality, an appeal which extends to Ephesians 5:21.

J Vernon McGee writes that...

We have seen the exhibition of the new man and the inhibition of the new man. Now we come to the prohibition of the new man. There is the negative side of the believer’s life, which I think is important for us to see. There is not enough emphasis on it. We talk about “new morality” which is nothing in the world but old sin. There is a liberty in Christ, but it is not a license to sin. (McGee, J V: Thru the Bible Commentary:  Thomas Nelson)

Say (3004) (lego) means to speak or talk, with apparent focus upon the content of what is said.

Affirm (3143) (marturomai from mártus = witness) (See related word studies - epimartureo;  diamarturomai) means to testify or bear witness.

The idea of marturomai is to bear witness with a solemn protestation, making an emphatic affirmation or a serious declaration (Acts 20:26, 26:22, Gal 5:3). To make a serious declaration on the basis of presumed personal knowledge

To affirm (state positively, assert as valid or confirmed, implying conviction based on evidence, experience or faith) something with solemnity (see NT uses below). The verb means to appeal to by something sacred. To urge as a matter of great importance and thus to affirm, insist or implore (Eph 4:17, 1Thes 2:11) To be emphatic in stating an opinion or desire.

It is used in the present verse to convey a solemn declaration of the nature of an appeal to God. What follows is of vital importance for the Gentile believers to hear for they still live within the cauldron of rank paganism and it's manifold and subtle temptations are prone to rear their ugly head.

Together (1722) (en) is actually the preposition meaning "in".

With the Lord identifies Paul with Christ and indicates he is giving the exhortation as if made by Christ Himself. Beloved, our by way of application, our Lord is clearly speaking to us who live in the midst of a society that is literally disintegrating morally and ethically. What a vital message to harken to!

By prefacing his exhortation with "I say and affirm together with the Lord" Paul is emphasizing the vital importance of what he is about to say. One feels here the tremendous burden upon Paul's heart to impress deeply upon those to whom he writes the imperative necessity of a revolutionary change in their whole manner of living. So his language corresponds with the truth he frankly and faithfully presents. By adding "together with the Lord" Paul would have his readers know that he is not stating personal conviction regarding the standard for their Christian life, but that it is the living Lord speaking through him. And so he speaks insistently, earnestly and emphatically saying in essence

"I and the Lord in me are solemnly declaring, so pay very close attention to what I am about to tell you."

As Wayne Barber says, Paul

wants them to see that Christianity is radically different from the way the world lives. These people had come out of the world. The temptation is always to go back to where we have come from. Paul says, "Oh no." He has given us a picture of what the Christian life is all about, being strengthened in the inner man by the Spirit of God. Now he is saying, "Don’t go back. Live differently. It is a radically different lifestyle that you have now as a believer." (Ephesians 4:17-19: A Brand New Way of Life)

What is the problem? Why do believers need this warning since we have been delivered from our past enslavement to the power of Sin (see notes on Romans 6:11; 6:12; 6:13; 6:14)?

The problem all believers must still contend with is what the Bible refers to as flesh (see note), that evil disposition inherited from Adam and which still resides in the mortal (physical) bodies even of believers. Peter for example is addressing believers and exhorts them...

"as aliens and strangers to (continually - present tense) abstain from fleshly lusts, which wage war (continually - present tense) against the soul." (see note 1 Peter 2:11)

Clearly Peter is implying that the flesh remains a force with which every believer must daily, earnestly contend. The flesh reflects what remains of the “old man” (inherited from  Adam see note Romans 5:12) and which still exists even after a person is saved. Flesh is that unredeemed part of a believer that awaits future redemption at the time of glorification (see note Romans 8:23). At that glorious time we will be completely free of not only the presence of sin but the "pleasure" of sin.  Flesh is that moral and spiritual weakness and helplessness of human nature that still clings to redeemed souls. In short, the flesh of Christians is that entity that remains within us that stimulates evil desires to commit trespasses and sins. As long as we inhabit these mortal bodies, we have to contend with the flesh which gives rise to deceitful lusts  or strong desires that ever tend to pull us back to the miry clay from which we were transferred by God when He took us from the kingdom of darkness and into His marvelous light, the kingdom of His beloved Son, in Whom we have redemption, the forgiveness of sins.

THAT YOU WALK NO LONGER JUST AS THE GENTILES ALSO WALK: meketi humas peripatein (PAN) kathos kai ta ethne peripatei (3SPAI):   (1 Thessalonians 4:1,2; 1 Timothy 5:21; 6:13; 2 Timothy 4:1)  (1:22; 2:1-3; 5:3-8; Romans 1:23-32; 1 Corinthians 6:9-11; Galatians 5:19-21; Colossians 3:5-8; 1 Peter 4:3,4)

You - refers to the saved Gentiles. In one sense yes they are still "Gentiles" but in the eternal sense, they belong to a new race, for they are individually each a new creation (2Cor 5:17) and corporately one new man (see notes Ephesians 2:15). In a sense then, there are not "3 races", Jews, Gentiles and Christians. In chapter 1 Paul had explained what transpired to transfer them from the Kingdom of darkness to the Kingdom of light writing...

"In Him [Jesus], you also, after listening to the message of the truth, of the gospel of your salvation—having also believed, you were sealed in Him with the Holy Spirit of promise." (See note Ephesians 1:13)

So now don't walk like you used to walk when you were pagan idol worshipping, God hating men and women. Don't do it! You are saved to live life on a high plane, and not in the "sewer" of godless men. Yes, you still live (temporarily) in this fallen, sin sick, morally decaying world but you have bee set free from it and are to be "lighthouses" permeating it with the light of Christ, not being drawn back into its moral mire and spiritual decay and darkness.

As Paul reminds us in Titus...

we also once were foolish ourselves, disobedient, deceived, enslaved to various lusts and pleasures, spending our life in malice and envy, hateful, hating one another. (See note Titus 3:3)

Just as the Gentiles also walk - Their new position in Christ was the fulcrum of Paul's argument for a new walk, for he knew that new practice must result from a new position. So with invincible logic he proceeds to call them to an altogether different walk from that of the unsaved Gentiles among whom they still lived. In chapter 2 Paul had repeatedly reminded the Gentiles of their former manner of walking writing...

And you were dead in your trespasses and sins, 2 in which you formerly walked according to the course of this world, according to the prince of the power of the air, of the spirit that is now working in the sons of disobedience. 3 Among them we too all formerly lived in the lusts of our flesh, indulging the desires of the flesh and of the mind, and were by nature children of wrath, even as the rest (See notes Ephesians 2:1; 2:2; 2:3) (Comment: They were dead toward God, alive to Satan's power leading them to disobey and were enslaved to the power of the wicked lusts of their flesh. And they had no power to change! Paul is saying don't walk that way anymore. You have been freed from that vile prison. Don't go back and put those chains on. Paul in his review of their spiritual history, what he wrote in chapter 2 was "then, but he is calling for them to live in the "now" of their new life in Christ.)

Wayne Barber paraphrases what Paul is saying...

"I have just raised you up to the highest level of understanding that you could possibly get to. Now I am warning you. Don’t you go back and live like you used to live. When you do, sin will take you further than you ever wanted to stray, keep you longer than you ever wanted to stay and cost you more than you ever dreamed you would pay." Imagine going back to live that way...It is amazing...how quickly we forget what has caused us so much pain. (Ephesians 4:17-19: A Brand New Way of Life)

As Ruth Paxson commenting on "walk no longer" writes that...

These words have an authoritative tone of finality about them. "Put off" demands unconditional renunciation (see notes Ephesians 4:22). The Christian has begun a walk on a new road in a new sphere leading to a new goal. Then he must be prepared at the very beginning with deliberate determination to make a full and final abandonment of the old life in its entirety. But a walk is taken step by step. So as one goes along the new road and recognizes soft spots in character, backslidings in conduct, danger points in companionships, discrepancies in ethics, departures in morals, and compromises in standard, there must be an immediate putting off of that old remnant of the abandoned life. Paul makes this quite clear in Chapters four and five, as he mentions definite sins still to be found in the lives of these Ephesian Christians. (Paxson, Ruth: The Wealth, the Walk and the Warfare of the Christian. 1939. Revell)

Gentiles (1484) (ethnos) in context refers to the unsaved pagan idol worshipers who were far off from God. They were no longer pagan Gentiles but a new race called "Christians", a new man who should demonstrate a clean cut cleavage from their former life of rank paganism. There was a moment in time in the past when they had crossed the boundary line from eternal death into eternal life. In that moment of spiritual rebirth something so tremendously revolutionary had taken place that the sinner had been made into a saint and thus life could never be as it was before.

Walk (4043) (peripateo from peri = about, around + pateo = walk, tread) (Click word study on peripateo) means literally to walk about here and there or to tread all around. Peripateo then came to mean, to make one’s way, to make progress, to make due use of one’s opportunities and finally (as used by Paul in Ephesians), to live, to regulate one’s life, to conduct one’s self. Most of the NT uses refer to the daily conduct of one's life or how one orders their behavior or passes their life. The present tense points to a habitual action - don't fall back into the habitual practices of those who do not know Christ as Lord.

Earlier Paul explained to the saints at Ephesus that it was in the sphere of trespasses and sins ...

in which you formerly walked (peripateo) according to the course of this world, according to the prince of the power of the air, of the spirit that is now working in the sons of disobedience. (See note Ephesians 2:2)

IN THE FUTILITY OF THEIR MIND: en mataioteti tou noos auton:  (Psalms 94:8-11; Acts 14:15)

Futility (3153) (mataiotes from mataios = vain, empty <> derived from maten = to no purpose or in vain) means emptiness, vanity, nonsense, nothingness! Thayer says mataiotes is a "purely Biblical and ecclesiastical word" which describes "what is devoid of truth and appropriateness". It defines the inability to reach a goal or achieve a purpose. Mataiotes describes the state of being without use or value, emptiness, futility, purposelessness, transitoriness. It has the quality of being empty, fruitless, nonproductive, useless. Mataiotes speaks of want of attainment with the idea of aimlessness or of leading to no object or end.

It is interesting to note that "vain things" was a Jewish name for the Gentile idols, which represented ideas and conceptions of a god that had no intrinsic value or correspondence to the real truth about the Living God.

The heathen are concerned with empty things which do not matter in the eternal scheme of things. Their mind was void of useful aims or goals (eternally speaking).

In Romans 1 we see how this futility of their mind was a consequence of their rejection of the truth about God...

For even though they knew God, they did not honor Him as God, or give thanks; but they became futile (passive voice) in their speculations, and their foolish heart was darkened. (See note Romans 1:21) (Comment: Unbelieving Gentiles rejected the truth about God and thus failed to attain the true purpose of the mind which is to receive God’s revelation which would have led them to see there is a Creator).

McGee comments that the futility of their mind...

means the empty illusion of the life that thinks there is satisfaction in sin. Oh, how many people walk that way! I feel so sorry for these young people who have been taken in by the promoters of immorality as a life style. (McGee, J V: Thru the Bible Commentary:  Thomas Nelson)

MacDonald puts it this way...

Their life was empty, purposeless, and fruitless. There was great activity but no progress. They chased bubbles and shadows, and neglected the great realities of life. (MacDonald, W & Farstad, A. Believer's Bible Commentary: Thomas Nelson)

 Matthew Poole in a commentary published in the 1680's writes that

Their minds themselves, and understandings, the highest and noblest faculties in them being conversant about things empty, transient, and unprofitable, and which deceive their expectations (are) therefore are vain, viz. their idols, their worldly enjoyment, etc.”

Vincent has an interesting note on mataiotes in (Romans 8:20) writing that...

Kenos (2756) signifies empty; mataios idle, resultless. Kenos, used of persons, implies not merely the absence of good, but the presence of evil. (See Ja 2:20). The Greek proverb runs: “The empty think empty things.” Mataios expresses aimlessness. All which has not God for the true end of its being is mataios . Pindar describes the vain man as one who hunts bootless things with fruitless hopes. Plato (“Laws,” 735) of labor to no purpose. Ezek. 13:6, “prophesying vain things (mataia),” things which God will not bring to pass. Compare note Titus 3:9. In Romans 8:20 the reference is to a perishable and decaying condition, separate from God, and pursuing false ends.

There are three uses of mataiotes in the NT (see the other 2 verses below) but some 46 verses (most in Ecclesiastes) in the Septuagint (LXX) (Ps 4:2; 26:4; 31:6; 38:12; 39:5; 40:4; 52:7; 62:9; 78:33; 119:37; 139:20; 144:4, 8, 11; Pr. 22:8; Eccl. 1:2, 14; 2:1, 11, 15, 17, 19, 21, 23, 26; 3:19; 4:4, 7-8, 16; 5:7, 10; 6:2, 4, 9, 11-12; 7:6, 15; 8:10, 14; 9:2, 9; 11:8, 10; 12:8). For example...

Ps 144:4 Man is like a mere breath (Lxx = mataiotes = nothingness, emptiness); His days are like a passing shadow.

The other 2 NT uses of mataiotes are...

Romans 8:20 (see note) For the creation was subjected to futility, not of its own will, but because of Him who subjected it, in hope

2 Peter 2:18 (see note) For speaking out arrogant words of vanity they entice by fleshly desires, by sensuality, those who barely escape from the ones who live in error,

Mind (3563) (nous) refers to the organ of mental perception and apprehension, of conscious life, of the consciousness preceding actions or recognizing and judging them. Nous represents the seat of understanding and intellect, the reasoning capacity or the thinking faculty. Believers have a new mind "the mind (nous) of Christ" (1 Cor 2:16) which can be renewed as they chose not to be conformed to this world's way of thinking but to be radically transformed (see expository note on Romans 12:2). The mind (nous) Paul is describing in this verse is what he refers to elsewhere as the "fleshly mind" or

Marvin Vincent  calls nous...

"the intellectual faculty in its moral aspects as determined by the fleshly, sinful nature" (see note Colossians 2:18)

Nous is the God given faculty of perceiving and understanding and is the channel through which truth reaches the heart. Paul says that believers "have the mind of Christ." (1Cor 2:16) Although present-day believers are typically not concerned with Jewish ritual observances, the principle is still applicable. We should be more concerned about renewing our mind and focusing it on Jesus than observing a list of rules that have no biblical support.

Paul's point is that when they were unregenerate Gentiles, they could understand spiritual truth. The way the pagan world thinks is totally foreign to they way God thinks. In fact, every person still spiritually dead in their trespasses and sins does not even have the capability to comprehend God. As Paul explained to the church at Corinth...

"a natural (unsaved, still "in Adam", not "in Christ") man does not accept (dechomai = deliberately and readily, receive kindly, they do not "put out a welcome mat"! = present tense) the things of the Spirit of God; for they are foolishness (moria = that which is considered intellectually weak, irrational) to him, and he cannot (dunamai = present tense = have intrinsic power - natural men lack the inner, inherent ability and resources on their own to) understand (verb ginosko = know by experience) them, because they are spiritually appraised (anakrino =  sift up and down and so to scrutinize, to examine accurately and carefully with exact research like in legal processes)." (1Cor 2:14)

In the next verse Paul explains why the unsaved man cannot comprehend the things of God.

Expositor's Greek Testament writes that...

“It is a description of the walk of the heathen world generally—a walk moving within the limits of intellectual and moral resultlessness, given over to things devoid of worth or reality.” (Nicoll, W Robertson, Editor: Expositors Greek Testament: 5 Volumes. Out of print. Search Google)