20 But you have not so learned Christ,
Beginning with this verse, Paul presented the “new man” to contrast with the old man’s characteristics set forth in verses 17 to 19. Verses 20 to 24 show what a genuine believer is.
The word “but” introduces a sharp contrast to the characteristics of the non-Christian in verses 17 to 19. Christians have an entirely different kind of life than unbelievers.
you have not so learned [come to understand] Christ,
When some Ephesians became believers, they no longer possessed a hard heart toward Christ or were primarily oriented to an impure lifestyle (Eph 4:11-19). They did not merely learn about Christ, but they embraced His very person and received new life from Him.
“Learned” in the Greek means they learned Christ at one point, the point of their salvation (aorist indicative). This is far more than learning doctrine about Christ; it is about coming to know Him personally.
The central lesson of Christianity is to know Christ and what He did.
The regenerate life of the believer is a Christ-centered life and operates with the ultimate purpose to glorify Him on earth. Our old man was our non-Christian life shamelessly oriented to sin and callousness toward God. The old man is deprived of a relationship to Christ, but the new man lives in an entirely new sphere created by God.
Knowledge of Christ is in its essence light or revelation from God. When it enters the born-again mind, it changes outlook on and behavior towards the world.