Ephesians 4:17

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,

Paul calls followers of Christ to be different than the world. The world (here signified Gentiles) walks in vanity of mind, or in foolishness or futility. Louw-Nida defines "vnity" as: pertaining to being useless on the basis of being futile and lacking in content—‘useless, futile, empty, futility.’ There thoughts are futile and without (true) content; they think wrongly, because their minds are blinded by Satan.

We (followers of Christ) are to be different. We are to walk with a mind that follows Christ, or as Paul succinctly puts it the truth is in Christ, vs. 21.