Ephesians 4:17

This I say therefore and testify in the Lord
These words may be considered either as an assertion, and so a testimonial of the different walk and conversation of the saints at Ephesus, from the rest of the Gentiles; or as an exhortation in the name of the Lord to such a walk, the apostle here returning to what he stirs them up to in ( Ephesians 4:1 )

that ye henceforth walk not as other Gentiles walk, in the vanity of
their mind;
every natural man walks in a vain show; the mind of man is vain, and whoever walk according to the dictates of it, must walk vainly: the phrase is expressive of the emptiness of the mind; it being naturally destitute of God, of the knowledge, fear, and grace of God; and of Jesus Christ, of the knowledge of him, faith in him, and love to him; and of the Spirit and his graces; and it also points at the instability and changeableness of the human mind, in which sense man at his best estate was altogether vanity; as also the folly, falsehood, and wickedness of it in his fallen state: and the mind discovers its vanity in its thoughts and imaginations, which are vain and foolish; in the happiness it proposes to itself, which lies in vain things, as worldly riches, honours and in the ways and means it takes to obtain it, and in words and actions; and the Gentiles showed the vanity of their minds in their vain philosophy and curious inquiries into things, and in their polytheism and idolatry: to walk herein, is to act according to the dictates of a vain and carnal mind; and it denotes a continued series of sinning, or a vain conversation maintained, a progress and obstinate persisting therein with pleasure: now God's elect before conversion walked as others do, but when they are converted their walk and conversation is not, at least it ought not to be, like that of others: the Alexandrian copy, and some others, the Vulgate Latin and Ethiopic versions, leave out the word "other", and only read, "as the Gentiles"