{4} Not in the lust of concupiscence, even as the Gentiles which know not God:

(4) The third, because the saints are distinguished by honesty and purity from those who do not know God.

People's New Testament

4:5 Even as the Gentiles which know not God. Even the greatest of heathen moralists, Socrates, instructed a harlot how she should conduct her shameful business. The heathen moralists condemned unchastity only in the case of a childbearing wife, as it would wrong her husband not to know the paternity of her children.

Wesley's Notes

4:5 Not in passionate desire - Which had no place in man when in a state of innocence. Who know not God - And so may naturally seek happiness in a creature. What seemingly accidental words slide in; and yet how fine, and how vastly important!

Jamieson-Fausset-Brown Bible Commentary

5. in the lust-Greek, "passion"; which implies that such a one is unconsciously the passive slave of lust.

which know not God-and so know no better. Ignorance of true religion is the parent of unchastity (Eph 4:18, 19). A people's morals are like the objects of their worship (De 7:26; Ps 115:8; Ro 1:23, 24).

Matthew Henry's Concise Commentary

4:1-8 To abide in the faith of the gospel is not enough, we must abound in the work of faith. The rule according to which all ought to walk and act, is the commandments given by the Lord Jesus Christ. Sanctification, in the renewal of their souls under the influences of the Holy Spirit, and attention to appointed duties, constituted the will of God respecting them. In aspiring after this renewal of the soul unto holiness, strict restraint must be put upon the appetites and senses of the body, and on the thoughts and inclinations of the will, which lead to wrong uses of them. The Lord calls none into his family to live unholy lives, but that they may be taught and enabled to walk before him in holiness. Some make light of the precepts of holiness, because they hear them from men; but they are God's commands, and to break them is to despise God.