Ephesians 4:17

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

Therefore - resuming the exhortation (Ephesians 4:1), 'I therefore beseech you that ye walk worthy,' etc.

Testify in the Lord - IN whom (as our element) we discharge our ministry (1 Thessalonians 4:1, Greek; Romans 9:1).

Henceforth walk not, [ meeketi (G3370)] - 'no longer;' resumed from Ephesians 4:14,

Other - Greek, 'the rest of the Gentiles.'

In the vanity ... - as their element: opposed to "in the Lord." 'Vanity of mind' (Romans 1:21) is the waste of the rational and moral powers on worthless objects, of which idolatry is a glaring instance. The root of it is departure from the knowledge of the trite God (Ephesians 4:18; 1 Thessalonians 4:5).