BUT YOU DID NOT LEARN CHRIST IN THIS WAY: humeis de ouch houtos emathete (2PAAI) ton Christon: (Luke 24:47; John 6:45; Romans 6:1,2; 2 Corinthians 5:14,15; Titus 2:11-14; 1 John 2:27) 

But you did not learn - The implication from his statement in verse 17 (walk no longer) is that some of his readers had drifted back into their old unregenerate ways. Paul is trying to correct their defective thinking (note the words that have to do with "thinking" - learn, heard, taught in next verse) and potentially destructive behavior and thus he begins by reminding them of how they had been delivered from the ethical sewer in which they had once wallowed.

John MacArthur rightly sounds the alarm writing that...

The ways of God and the ways of the world are not compatible. The idea, promoted by some who claim to be evangelicals, that a Christian does not have to give up anything or change anything when he becomes a Christian is nothing less than diabolical. That notion, under the pretense of elevating God’s grace and of protecting the gospel from works righteousness, will do nothing but send many people confidently down the broad way that Jesus said leads to destruction (see note Matthew 7:13). (MacArthur, J: Ephesians. Chicago: Moody Press)

But (de) draws a contrast with the old self described in verses 17-19.

You (humeis)- Refers to "you, the saved Gentiles" and is first in the Greek text presenting a sharp (emphatic) contrast with the pagan lifestyle of the Gentiles who were yet unconverted.

Not (3756) (ou) means absolutely not. In contrast to the insensitive, passion-dominated pagans who lived to satisfy their lower nature, this was not how the Gentile converts in Ephesus came to know Christ for themselves.

Learn (3129) (manthano) means to gain knowledge or skill by instruction. It conveys a basic meaning of directing one’s mind to something and producing an external effect.  Depending on the context, the learning can be through instruction, through inquiry (ascertain, discover, find out), through practice or experience (come to know, come to realize) or as achieving comprehension (understand). The idea is to genuinely understand and accept a teaching accept it as true and to apply it in one’s life.

When did they learn Christ? The aorist tense marks a specific time which would equate with the time of their conversion. They did not learn to follow Christ by the natural mental processes that customarily lead to the degradation of unsaved Gentiles. They learned to follow Him as His disciples from the gospel.  Christ teaches men to renounce sin and vice and to cultivate holiness and virtue.

Vincent says

“The phrase learn Christ occurs nowhere else. Christ does not stand for the doctrine of Christ; but Christ is the subject of His own message. (Ephesians 4)

TDNT adds that manthano is derived

From the basic sense “to direct one’s mind to something,” manthano comes to be used for (1) “to accustom oneself to something,” (2) “to experience,” (3) “to learn to know,” (4) “to understand,” (5) “to learn under instruction,” and (6) “to receive direction from a deity by oracle.” In the phrase tí mathon (7) it means “why?” (often with an ironical note). The use consistently implies an intellectual process that always has external effects and involves a conscious or unconscious intellectual initiative. Hence other terms may elucidate it but cannot replace it. (Kittel, G., Friedrich, G., & Bromiley, G. W.  Theological Dictionary of the New Testament. Eerdmans)

Christ (5547) (Christos from chrio = to anoint, rub with oil, consecrate to an office) is the Anointed One, the Messiah, Christos being the Greek equivalent of the transliterated Hebrew word Messiah. As a Jew learned the Torah, now the Christian learns Christ!

Christ, the Messiah, of Whom these Gentiles formerly did not even have knowledge of ("separate from Christ" see note Ephesians 2:12) was now He Who was the very essence of the content of the preaching which they had heard, the sum of the instruction they had received and the knowledge they had gained at the time of their conversion.

Paul wrote that...

indeed Jews ask for signs, and Greeks search for wisdom; but we preach Christ crucified, to Jews a stumbling block, and to Gentiles foolishness, but to those who are the called, both Jews and Greeks, Christ the power of God and the wisdom of God. (1Corinthians 1:22-24)

For I determined to know nothing among you except Jesus Christ, and Him crucified. (1Cor 2:2)

IF INDEED YOU HAVE HEARD HIM AND HAVE BEEN TAUGHT IN HIM: ei ge auton ekousate (2PAAI) kai en auto edidachthete, (2PAPI):  (Matthew 17:5; Luke 10:16; John 10:27; Acts 3:22,23; Hebrews 3:7,8) 

If indeed - two particles ei (1487) = if +  ge (1065) = indeed. It can be translated "seeing that" or "since" and when used with the indicative this conditional clause is viewed as fulfilled. In other words, what follows is not in any doubt but is taken for granted.

Expositors Greek Testament comments...

The point, therefore, is this—‘if, as I take it to be the fact, it was He, the Christ, that was the subject and the sum of the preaching which you heard then.’” (Nicoll, W Robertson, Editor: Expositors Greek Testament: 5 Volumes. Out of print. Search Google)

Heard (191) (akouo) means to hear with attention, with the ear of the mind or effectually so as to respond appropriately  to what has been spoken or taught.

McGee writes that...

If anyone is not listening to Jesus, then Jesus must not be his Savior. The Lord Jesus is the Shepherd and His sheep hear His voice. If you haven’t heard His voice, then you are not one of His sheep. What will change the Gentiles from their old nature? What are they to do? They are to listen to Christ. They are to hear Him. They are to be taught by Him. Those who are not His sheep will not hear Him. (McGee, J V: Thru the Bible Commentary:  Thomas Nelson)

Earlier Paul had written...

In Him (Christ), you also, after listening (akouo - hearing) to the message of truth, the gospel of your salvation-- having also believed, you were sealed in Him with the Holy Spirit of promise  (See note Ephesians 1:13)

Have been taught (1321) (didasko from dáo= know or teach) means to provide instruction in a formal or informal setting. Didasko refers to imparting positive truth. It is the responsibility of every believer (Col 3:16), and is part of the Great Commission (Mt 28:20). It is especially the responsibility of church leaders. “An overseer, then, must be… able to teach” (1Ti 3:2). Heresy flourishes where sound Christian teaching lags. The idea is to hold discourse with others in order to instruct them by word of mouth (tutor, direct, advise, put in mind). In the NT almost without exception didasko refers to the teaching of groups.

Didasko means to teach a student in such a way that the will of the student becomes conformed to the teaching taught. So the teacher teaches in such a way that as the student is taught, he now changes his mind saying in essence ''I won't do it this way, but I will do it this way because I've learned this doctrine or this teaching.'' Doctrine determines direction of our behavior, conformed to world or to God?

Teaching that Scripture finds significant is not that which provides information alone but also the teaching that creates disciples who live in responsive obedience to God's will.

In Him - speaks of their union with Christ ("in union with Christ" or "in fellowship with Him")

Expositors Greek Testament adds that

en autoi (in Him) is not to be reduced to ‘by Him,’ or ‘about Him,’ or ‘in His name,’ but has its proper sense of ‘in Him.’ The underlying idea is that of union with Christ. The ‘taught,’ therefore, refers probably to instructions subsequent to those which were given them at their first hearing. It was in fellowship with Christ that they received these instructions.” (Nicoll, W Robertson, Editor: Expositors Greek Testament: 5 Volumes. Out of print. Search Google)

JUST AS TRUTH IS IN JESUS: kathos estin (3SPAI) aletheia en to Iesou: (1:13; Psalms 45:4; 85:10,11; John 1:17; 14:6,17; 2 Corinthians 1:20; 11:10; 1 John 5:10-12,20)

Just as the truth is in Jesus - Vincent comments that

Just as corresponds with not so. Ye did not in such a manner learn Christ if ye were taught in such a manner as is truth, etc. Render, as Rev., as truth is in Jesus. Schaff paraphrases:

‘If you were taught so that what you received is true as embodied in the personal Saviour.’

‘Taught in the lines of eternal fact and spiritual reality which meet in Him’ (Moule).

Jesus is used rather than Christ; the historical rather than the official name. The life of Christianity consists in believing fellowship with the historic Jesus, Who is the Christ of prophecy.”  (Ephesians 4)

Truth (225) (aletheia from a = without + lêthô = that which is hidden or concealed, the combination meaning out in open) is the  the unconcealed reality lying at the basis of and agreeing with an appearance; the manifested, the veritable essence of matter. John writes that...

For the Law was given through Moses; grace and truth were realized through Jesus Christ. (John 1:17)

Jesus said to him, "I am the way, and the truth, and the life; no one comes to the Father, but through Me. (John 14:6)

Blaikie remarks that

All truth acquires a different hue and a different character when there is a personal relation to Jesus. Truth apart from the Person of Christ has little power. (Pulpit Commentary. Ephesians. Ages Software)

TDNT explains the origin of this word this way...

Etymologically aletheia means “nonconcealment.” It thus denotes what is seen, indicated, expressed, or disclosed, i.e., a thing as it really is, not as it is concealed or falsified. aletheia is “the real state of affairs,” e.g., the truth in law, or real events in history, or true being in philosophy... aletheia is “that which has certainty and force”... aletheia is “that on which one can rely”...aletheia is “the state of affairs as disclosed”... aletheia is “truth of statement” used with speaking (Lk. 4:25) or teaching (Mk. 12:14).... aletheia is “true teaching or faith” (2 Cor. 13:8; 4:2; Gal. 5:7; 1 Pet. 1:22)  (Kittel, G., Friedrich, G., & Bromiley, G. W.  Theological Dictionary of the New Testament. Eerdmans)

Truth then is the correspondence between a reality and a declaration which professes to set it forth. To say it another way, words are true when they correspond with objective reality. Persons and things are true when they correspond with their profession. Hence a truth is a declaration which has corresponding reality, or a reality which is correctly set forth. Since God is Himself the great reality, that which correctly sets forth His nature is pre-eminently the Truth.

Truth is the property of being in accord with fact or reality as defined by God. Whatever God says is Truth. Truth is a person, Jesus.

Jesus (2424) (Iesous) is transliterated from the Hebrew Yeshua which means Jehovah is help or Jehovah is Salvation. Jesus corresponds to the OT name Jehoshua is contracted to Joshua.

MacDonald makes an excellent point on why Paul would use the specific name of Jesus in this context writing that...

The name Jesus takes us back to His life on earth, since that is His name in Incarnation. In that spotless life which He lived as a Man in this world, we see the very antithesis of the walk of the Gentiles which Paul has just described. (MacDonald, W & Farstad, A. Believer's Bible Commentary: Thomas Nelson)

THAT, IN REFERENCE TO YOUR FORMER MANNER OF LIFE: kata ten proteran anastrophen:  (Eph 4:25; 1 Samuel 1:14; Job 22:23; Ezekiel 18:30-32; Colossians 2:11; 3:8,9; Hebrews 12:1; James 1:21; 1 Peter 2:1,2)  (17; 2:3; Galatians 1:13; Colossians 3:7; 1 Peter 1:18; 4:3; 2 Peter 2:7) 

Former (4387) (próteros from pró = forth, before) means prior, previous, of an earlier time, all pertaining to a point of time earlier in a sequence.

In chapter 2 Paul had repeatedly reminded them of their former desperate spiritual condition 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)

Therefore remember, that formerly you, the Gentiles in the flesh, who are called "Uncircumcision" by the so-called "Circumcision," which is performed in the flesh by human hands--12 remember that you were at that time separate from Christ, excluded from the commonwealth of Israel, and strangers to the covenants of promise, having no hope and without God in the world.13 But now in Christ Jesus you who formerly were far off have been brought near by the blood of Christ. (See notes Ephesians 2:11; 2:12; 2:13)

Manner of life (391) (anastrophe from aná = again, back + strépho = turn - idea is turning back in forth in a place equates with living there) means ones way of life or conduct, with apparent focus on overt daily behavior. Thayer adds that the root verb (anastrepho) means “to conduct or behave one’s self, to walk,” the latter meaning not referring here to the physical act of walking but to the act of determining our course of conduct and the carrying out of that determined course of action. Anastrophe means in biblical use has the moral and spiritual aspect of one’s manner of life is in view.

Vincent comments that...

The process of development in the meaning of the word is interesting. 1. A turning upside down. 2. A turning about or wheeling. 3. Turning about in a place, going back and forth there about one’s business; and so, 4, one’s mode of life or conduct. (Word studies in the New Testament)

YOU LAY ASIDE THE OLD SELF: apothesthai (AMN)...ton palaion anthropon ton:  (Romans 6:6; Colossians 3:9)

See related resource: Covenant: The Exchange of Robes - Putting Off the Old Man, Putting on the New Man

You lay aside the old self - without being too technical, the verb lay aside can be translated one of two ways, either indicating a completed past action (this would be our position whereas the second translation reflects our practice) or an action the believer is to carry out (the latter almost giving it the sense of an imperative). This distinction is even seen in the way the various Bible translations render the Greek text.

For example, the following rendering translates the Greek as if it were a fact or a past completed action...

Wuest: that you have put off once for all with reference to your former manner of life the old self who is being corrupted according to the passionate desires of deceit;

In contrast, the following versions render lay aside (put off, throw off, strip) as an action the believer is to carry out...

Amplified:  Strip yourselves of your former nature [put off and discard your old unrenewed self] which characterized your previous manner of life and becomes corrupt through lusts and desires that spring from delusion;

KJV: That ye put off concerning the former conversation the old man, which is corrupt according to the deceitful lusts;

NASB: that, in reference to your former manner of life, you lay aside the old self, which is being corrupted in accordance with the lusts of deceit,

NLT: throw off your old evil nature and your former way of life, which is rotten through and through, full of lust and deception.

I favor the interpretation of lay aside as an action the believer is to carry out and that is how it is presented in these notes. However, in one sense both interpretations/translations are correct, the former (positional truth) emphasizing the bestowal of God's sovereign grace in salvation and the latter (practical truth) emphasizing man's responsibility to walk in the light (and power) of the grace and the Spirit bestowed.

J Vernon McGee agrees with this approach writing that...

We are to put off the old man and put on the new man in the same manner that we change our clothes. It is like putting off an old and unclean garment and then putting on a garment that is new and clean. The putting off the old man and putting on the new man cannot be done by self-effort, nor can it be done by striving to imitate Christ’s conduct. It has been done for the believing sinner by the death of Christ. We are like babes who cannot dress themselves. I have learned with my little grandson that a child doesn’t do very well when he tries to dress himself. As Christians we never reach the place where we can do that, and we don’t need to try. It already has been done for us. We are told in the Epistle to the Romans that the old man has already been crucified in the death of Christ.

“Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin” (see note Romans 6:6).

In view of the truth that the old man has already been crucified with Christ, we are to put it off in the power of the Holy Spirit. This does not mean that the flesh, the old nature, is ever eliminated in this life. We do not get rid of the old nature, but we are not to live in it; that is, we are not to allow it to control our lives.

On the other hand, we do have a new nature. This is the result of regeneration by the Holy Spirit. Any man in Christ is a new creature. We are to live in that new nature, that new man. This is a repetition of the great message of Romans. (McGee, J V: Thru the Bible Commentary:  Thomas Nelson)

Ruth Paxson writes that...

Ephesians 4:22 "That ye put off concerning the former conversation the old man, which is corrupt according to the deceitful lusts."

Ephesians 4:24 "And that ye put on the new man, which after God is created in righteousness and true holiness."

This twofold clear, crisp exhortation marks the meeting-point between God's part and ours in our sanctification. It is the crossroads between God's sovereign work through grace and man's cooperative action through faith. It is an exhortation to practical holiness in every phase of one's daily life.

Paul takes us immediately to the very source of life in each sphere and shows us two things. The character of life is due to its source and the character determines the conduct.

Life in the old sphere is tracked to its source, "the old man," and the conduct is corrupt because the character is such.

Life in the new sphere is traced to its source, "the new man," and the conduct is righteous and holy because the character is so.

The fountain determines the flow. Let us then consider these two fountain-heads and our responsibility in relation to them. (Paxson, Ruth: The Wealth, the Walk and the Warfare of the Christian. 1939. Revell)

Lay aside (659) (apotithemi from apo = away from, marker of dissociation, implying a rupture from a former association, separation, departure, cessation, any separation of one thing from another by which the union or fellowship of the two is destroyed  + tithemi = put, place) (Click word study on apotithemi) means literally to put or take something away from its normal location and put it out of the way. It was used literally of runners who participated in the Olympic games who cast off their clothes and running nearly completely naked in the stadium.

"The night is almost gone, and the day is at hand. Let us therefore lay aside the deeds of darkness and put on the armor of light." (see note)

"But now you also, put them all aside: anger, wrath, malice, slander, and abusive speech from your mouth."  (see note)

"Therefore, since we have so great a cloud of witnesses surrounding us, let us also lay aside every encumbrance, and the sin which so easily entangles us, and let us run with endurance the race that is set before us." (see note)

"Therefore, putting aside all malice and all guile and hypocrisy and envy and all slander" (see note)