“This I say therefore, and testify in the Lord, that ye henceforth walk not as other Gentiles walk, in the vanity of their mind,”
King James Version (KJV)
4:17 This therefore I say - He returns thither where he begun, #Eph 4:1|. And testify in the Lord - In the name and by the authority of the Lord Jesus. In the vanity of their mind - Having lost the knowledge of the true God, #Rom 1:21|. This is the root of all evil walking.
Eph 4:17 This I say, therefore. Now, after the statement of the great principles named in Eph 4:3-16, Paul returns to the exhortation of Eph 4:1-3. Walk not as other Gentiles. Having come out from the heathen and abandoned all their sins. The vanity of their mind. Vanity of mind means what is a waste of life.