"This I say . . . that ye henceforth walk not as other Gentiles walk." (Ephesians 4:17)
This succinct command quickly is followed by a sweeping description of the impotent mind of the Gentiles of that day in contrast to the utterly changed condition of the believer (whether Jew or Gentile). The non-Christian Gentiles had a darkened perceptive ability, rendering them alienated because of the ignorance that was in them, and an overall blindness of their heart which was the root cause of their inability to function, even to feel, in the same way as the children of God (Ephesians 4:18-19; compare Romans 1:21-32; 2 Corinthians 4:3-4).
The saint of God, however, is told to discard the old man and to put on the new man (Ephesians 4:20-24), as though that simple picture of a powerful reality is adequate instruction to fulfill the earlier command. No longer is the child of God to be corrupt by the deceitful lusts of his or her old condition, but having learned Christ and been taught by Him, is to be renewed in the spirit of (their) mind. A transformation is now possible through the new mental (intellectual, spiritual) abilities given to us by Christ (Romans 12:1-2; 1 Corinthians 2:16).
The new man, which we are responsible to wear like a body-enveloping cloak, is created for us by the omniscient Creator in righteousness and true holiness. We have been given a specially created new man to wear (externally visible), which will show (exhibit, demonstrate, make clear) the spiritual difference between the Gentiles and the saints of God.
The seventeen commands which follow (Ephesians 4:24-5:7) address every aspect of the Christian walk, all relating to a lifestyle of truth, giving specific contrast between the unbelieving Gentile and the saint. HMM III