Verse 21. Because that. The apostle here is showing that it was right to condemn men for their sins. To do this it was needful to show them that they had the knowledge of God, and the means of knowing what was right; and that the true source of their sins and idolatries was a corrupt and evil heart.

When they knew God. Greek, knowing God. That is, they had an acquaintance with the existence and many of the perfections of one God. That many of the philosophers of Greece and Rome had a knowledge of one God, there can be no doubt. This was undoubtedly the case with Pythagoras, who had travelled extensively in Egypt, and even in Palestine; and also with Plato and his disciples. This point is clearly shown by Cudworth in his Intellectual System, and by Bishop Warburton in the Divine Legation of Moses. Yet the knowledge of this great truth was not communicated to the people. It was confined to the philosophers; and not improbably one design of the mysteries celebrated throughout Greece was to keep up the knowledge of the one true God. Gibbon has remarked, that "the philosophers regarded all the popular superstitions as equally false; the common people as equally true; and the politicians as equally useful." This was probably a correct account of the prevalent feelings among the ancients. A single extract from Cicero (de Natura Deerum, lib. fl. e.6) will show that they had the knowledge of one God: "There is something in the nature of things Which the mind of man, which reason, which human power cannot effect; and certainly that which produces this must be better than man. What can this be called but God?" Again (c.2,) "What can be so plain and manifest, when we look at heaven, and contemplate heavenly things, as that there is some Divinity of most excellent mind, by which these things are governed?"

They glorified him not as God. They did not honour him as God. This was the true source of their abominations, To glorify him as God is to regard with proper reverence all his perfections and laws; to venerate his name, his power, his holiness, and presence, etc. As they were not inclined to do this, so they were given over to their own vain and wicked desires. Sinners are not willing to give honour to God as God. They are not pleased with his perfections; and therefore the mind becomes fixed on other objects, and the heart gives free indulgence to its own sinful desires. A willingness to honour God as God -- to reverence, love, and obey him, would effectually restrain men from sin.

Neither were thankful. The obligation to be thankful to God for his mercies, for the goodness which we experience, is plain and obvious. Thus we judge of favours received of our fellow-men. The apostle here clearly regards this unwillingness to render gratitude to God for his mercies as one of the causes of their subsequent corruption and idolatry. The reasons of this are the following.

(1.) The effect of ingratitude is to render the heart hard and insensible.

(2.) Men seek to forget the Being to whom they are unwilling to exercise gratitude.

(3.) To do this, they fix their affections on other things; and hence the heathen expressed their gratitude not to God, but to the sun, and moon, and stars, etc., the mediums by which God bestows his favours on men. And we may here learn, that an unwillingness to thank God for his mercies is one of the most certain causes of alienation and hardness of heart.

But became vain. To become vain, with us, means to be elated, or self-conceited, or to seek praise from others. The meaning here seems to be, they became foolish, frivolous ill their thoughts and reasonings. They acted foolishly; they employed themselves in useless and frivolous questions, the effect of which was to lead the mind farther and farther from the truth respecting God.

Imaginations. This word means, properly, thoughts; then reasonings, and also disputations. Perhaps our word speculations, would convey its meaning here. It implies that they were unwilling to honour him, they commenced those speculations which resulted in all their vain and foolish opinions about idols, and the various rites of idolatrous worship. Many of the speculations and inquiries of the ancients were among the most vain and senseless which the mind can conceive.

And their foolish heart. The word heart is not infrequently used to denote the mind, or the understanding. We apply it to denote the affections. But such was not its common use among the Hebrews. We speak of the head when we refer to the understanding, but this was not the case with the Hebrews. They spoke of the heart in this manner, and in this sense it is clearly used in this place. See Eph 1:18; Ro 2:15; 2 Co 4:6; 2 Pe 1:19.

The word foolish means, literally, that which is without understanding, Mt 15:16.

Was darkened. Was rendered obscure, so that they did not perceive and comprehend the truth. The process which is stated in this verse is,

(1.) that men had the knowledge of God;

(2.) that they refused to honour him when they knew hun, and were opposed to his character and government;

(3) that they were ungrateful;

(4) that they then began to doubt, to reason, to speculate, and wandered far into darkness. This is substantially the process by which men wander away from God now. They have the knowledge of God, but they do not love him; and being dissatisfied with his character and government, they begin to speculate, fall into error, and then "find no end in wandering mazes lost," and sink into the depths of heresy and of sin.

{b} "vain" Jer 2:5; Eph 4:17,18