{12} This I say therefore, and testify in the Lord, that ye henceforth walk not as other Gentiles walk, in the {z} vanity of their mind,

(12) He descends to the fruits of Christian doctrine, and reasons first upon the principles of conduct and actions, setting down a most grave comparison between the children of God, and those who are not regenerated. For in these men all the powers of the mind are corrupted, and their mind is given to vanity, and their senses are darkened with most gross mistiness, and their affections are so accustomed by little and little to wickedness, that at length they run headlong into all uncleanness, being utterly destitute of all judgment.

(z) If the noblest parts of the soul are corrupted, what is man but solely corruption?

People's New Testament

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.

Wesley's Notes

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.

Jamieson-Fausset-Brown Bible Commentary

17. therefore-resuming the exhortation which he had begun with, "I therefore beseech you that ye walk worthy," &c. (Eph 4:1).

testify in the Lord-in whom (as our element) we do all things pertaining to the ministry (1Th 4:1 [Alford]; Ro 9:1).

henceforth . not-Greek, "no longer"; resumed from Eph 4:14.

other-Greek, "the rest of the Gentiles."

in the vanity, &c.-as their element: opposed to "in the Lord." "Vanity of mind" is the waste of the rational powers on worthless objects, of which idolatry is one of the more glaring instances. The root of it is departure from the knowledge of the true God (Eph 4:18, 19; Ro 1:21; 1Th 4:5).

Matthew Henry's Concise Commentary

4:17-24 The apostle charged the Ephesians in the name and by the authority of the Lord Jesus, that having professed the gospel, they should not be as the unconverted Gentiles, who walked in vain fancies and carnal affections. Do not men, on every side, walk in the vanity of their minds? Must not we then urge the distinction between real and nominal Christians? They were void of all saving knowledge; they sat in darkness, and loved it rather than light. They had a dislike and hatred to a life of holiness, which is not only the way of life God requires and approves, and by which we live to him, but which has some likeness to God himself in his purity, righteousness, truth, and goodness. The truth of Christ appears in its beauty and power, when it appears as in Jesus. The corrupt nature is called a man; like the human body, it is of divers parts, supporting and strengthening one another. Sinful desires are deceitful lusts; they promise men happiness, but render them more miserable; and bring them to destruction, if not subdued and mortified. These therefore must be put off, as an old garment, a filthy garment; they must be subdued and mortified. But it is not enough to shake off corrupt principles; we must have gracious ones. By the new man, is meant the new nature, the new creature, directed by a new principle, even regenerating grace, enabling a man to lead a new life of righteousness and holiness. This is created, or brought forth by God's almighty power.