Eph. 4:17 This therefore I say and testify in the Lord, that you no longer walk as the Gentiles also walk in the vanity of their mind.
20-21 But you did not so learn Christ, if indeed you have heard Him and have been taught in Him as the reality is in Jesus.
The Gentiles are the fallen people, who become vain in their reasonings (Rom. 1:21). They walk without God in the vanity of their mind, being controlled and directed by their vain thoughts. Whatever they do according to their fallen mind is vanity, void of reality…. Paul’s exhortation in Ephesians 4:17 suggests that we the believers, who once walked in the vanity of the mind before we were saved, may continue to do so after we are saved. Even religious activities and practices among Christians which do not express God—such as Christmas and Easter—are superstition, falsehood, deception, and vanity. As believers, we should no longer walk in the vanity of the mind. Instead, we should allow the new man to live out God; every detail of the living of the new man must be God expressed in our living. This is to no longer walk in the vanity of the mind but to live the life of the new man as the reality is in Jesus. (The Conclusion of the New Testament, p. 3428)
In Ephesians 4:17-32 we see that Christ is the reality and grace for the living of the new man….When God in Christ was incarnated as a man, He came with grace and reality; that is, when Christ came, grace and reality came with Him. [In John 1:14 and 17] we see that grace and reality are personified in Jesus Christ; grace and reality refer to a person, the Triune God incarnated and expressed in humanity.
Ephesians presents reality and grace as a pair for the living of the new man. Ephesians 2:15 tells us that Christ created the one new man; the new man is on the earth today. Ephesians 4:17-32 goes on to reveal that the way for the new man to live is by reality and grace.
In Ephesians 4:17-21 and 24, Paul presents the reality in Jesus for a walk that is no longer in the vanity of the mind.
[In verse 21] Paul does not say that the reality is in Christ; rather, he says that the reality is in Jesus. The reason for this is that when the Lord Jesus lived on earth as a man, in Him there was always the reality. In Jesus, that is, in His human living and His daily walk—whether He worked as a carpenter or carried out His ministry—there was the reality at all times. In order to understand the reality expressed in the living of Jesus as a man on earth, we need to read verse 17, where Paul exhorts the believers to “no longer walk as the Gentiles also walk in the vanity of their mind.” Here we see a contrast between the reality in Jesus and the vanity of the Gentiles’ mind. In the human living of Jesus there was no vanity but only the reality; however, in our godless society there is nothing but vanity of vanities. Using a phrase from John Bunyan’s The Pilgrim’s Progress, we may say that, as a whole, today’s world is a “vanity fair.” Wealth, fame, position, entertainment, and material possessions in today’s world are all vanities, but everything in the daily living of the man Jesus is the reality. Whatever He did in His human living is the reality; not one of His words was vain (cf. 1 Pet. 2:22).
Christ is not only life to us but also an example (John 13:15; 1 Pet. 2:21). In His life on earth He set up a pattern, as revealed in the Gospels. Then He was crucified and resurrected to become the life-giving Spirit that He might enter into us to be our life. We learn from Him (Matt. 11:29) according to His example, not by our natural life but by Him as our life in resurrection. To learn Christ is simply to be molded into the pattern of Christ, that is, to be conformed to the image of Christ (Rom. 8:29). (The Conclusion of the New Testament, pp. 3425-3426)
Further Reading: The Conclusion of the New Testament, msgs. 118, 253, 341; CWWL, 1964, vol. 4, “Practical Lessons on the Experience of Life,” ch. 5