1:18-25 The apostle begins to show that all mankind need the salvation of the gospel, because none could obtain the favour of God, or escape his wrath by their own works. For no man can plead that he has fulfilled all his obligations to God and to his neighbour; nor can any truly say that he has fully acted up to the light afforded him. The sinfulness of man is described as ungodliness against the laws of the first table, and unrighteousness against those of the second. The cause of that sinfulness is holding the truth in unrighteousness. All, more or less, do what they know to be wrong, and omit what they know to be right, so that the plea of ignorance cannot be allowed from any. Our Creator's invisible power and Godhead are so clearly shown in the works he has made, that even idolaters and wicked Gentiles are left without excuse. They foolishly followed idolatry; and rational creatures changed the worship of the glorious Creator, for that of brutes, reptiles, and senseless images. They wandered from God, till all traces of true religion must have been lost, had not the revelation of the gospel prevented it. For whatever may be pretended, as to the sufficiency of man's reason to discover Divine truth and moral obligation, or to govern the practice aright, facts cannot be denied. And these plainly show that men have dishonoured God by the most absurd idolatries and superstitions; and have degraded themselves by the vilest affections and most abominable deeds.

21. Because that, when they knew God—that is, while still retaining some real knowledge of Him, and ere they sank down into the state next to be described.

they glorified him not as God, neither were thankful—neither yielded the adoration due to Himself, nor rendered the gratitude which His beneficence demanded.

but became vain—(compare Jer 2:5).

in their imaginations—thoughts, notions, speculations, regarding God; compare Mt 15:19; Lu 2:35; 1Co 3:20, Greek.

and their foolish—"senseless," "stupid."

heart—that is, their whole inner man.

was darkened—How instructively is the downward progress of the human soul here traced!