For when they knew God, etc. He plainly testifies here, that God has presented to the minds of all the means of knowing him, having so manifested himself by his works, that they must necessarily see what of themselves they seek not to know -- that there is some God; for the world does not by chance exist, nor could it have proceeded from itself. But we must ever bear in mind the degree of knowledge in which they continued; and this appears from what follows. They glorified him not as God. No idea can be formed of God without including his eternity, power, wisdom, goodness, truth, righteousness, and mercy. His eternity appears evident, because he is the maker of all things -- his power, because he holds all things in his hand and continues their existence -- his wisdom, because he has arranged things in such an exquisite order -- his goodness, for there is no other cause than himself, why he created all things, and no other reason, why he should be induced to preserve them -- his justice, because in his government he punishes the guilty and defends the innocent -- his mercy, because he bears with so much forbearance the perversity of men -- and his truth, because he is unchangeable. He then who has a right notion of God ought to give him the praise due to his eternity, wisdom, goodness, and justice. Since men have not recognized these attributes in God, but have dreamt of him as though he were an empty phantom, they are justly said to have impiously robbed him of his own glory. Nor is it without reason that he adds, that they were not thankful, [1] for there is no one who is not indebted to him for numberless benefits: yea, even on this account alone, because he has been pleased to reveal himself to us, he has abundantly made us indebted to him. But they became vain, [2] etc.; that is, having forsaken the truth of God, they turned to the vanity of their own reason, all the acuteness of which is fading and passes away like vapor. And thus their foolish mind, being involved in darkness, could understand nothing aright but was carried away headlong, in various ways, into errors and delusions. Their unrighteousness was this -- they quickly choked by their own depravity the seed of right knowledge, before it grew up to ripeness.

Footnotes

1 - The conjunctive, e, is for oute, says Piscator: but it is a Hebraism, for v is sometimes used in Hebrew without the negative, which belongs to a former clause. -- Ed.

2 - The original words are, emataiothesan en tois dialogismois auton, "Vani facti sunt in ratiocinationibus suis -- they became vain in their reasonings" Pareus, Beza, Turrettin, and Doddridge; "They became foolish by their own reasonings," Macknight "Whatever the right reason within," says Pareus, "or the frame of the world without, might have suggested respecting God, they indulged in pleasing speculations, specious reasonings, and in subtle and frivolous conclusions; some denied the existence of a God, as Epicurus and Democritus -- others doubted, as Protagoras and Diagoras -- others affirmed the existence of many gods, and these, as the Platonics, maintained that they are not corporeal, while the Greeks and Romans held them to be so, who worshipped dead men, impious, cruel, impure, and wicked. There were also the Egyptians, who worshipped as gods, brute animals, oxen, geese, birds, crocodiles, yea, what grew in their gardens, garlic's and onions. A very few, such as Plato and Aristotle, acknowledged one Supreme Being; but even these deprived him of his providence. These, and the like, were the monstrous opinions which the Gentiles deduced from their reasonings. They became vain, foolish, senseless." "And darkened became their foolish heart," -- he asunetos auton kardia; "Corinthians eorum intelligentia carens -- their heart void of understanding;" "their unintelligent heart," Doddridge. Perhaps "undiscerning heart" would be the most suitable. See Matthew 15:16. Heart, after the manner of the Hebrews, is to be taken here for the whole soul, especially the mind. -- Ed.

Because that - The apostle here is showing that it was right to condemn people 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 traveled 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 Dr. 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 Deorum, lib. ii. c. 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 what 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 "honor" 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 honor 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 honor God as God - to reverence, love, and obey him, would effectually restrain people 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 favors 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) people 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 pagan expressed their gratitude not to God, but to the sun, and moon, and stars, etc., the mediums by which God bestows his favors upon people. 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 to be self-conceited, or to seek praise from others. The meaning here seems to be, they became foolish, frivolous in their thoughts and reasonings. They acted foolishly; they employed themselves in useless and frivolous questions, the effect of which was to lead the mind further and further 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 honor God, and being unwilling to honor 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 Ephesians 1:18; Romans 2:15; 2-Corinthians 4:6; 2-Peter 1:19. The word "foolish" means literally what is without "understanding;" Matthew 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 people had the knowledge of God.
(2) that they refused to honor him when they knew him, 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 people 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.

Because that when they knew God - When they thus acquired a general knowledge of the unity and perfections of the Divine nature, they glorified him not as God - they did not proclaim him to the people, but shut up his glory (as Bishop Warburton expresses it) in their mysteries, and gave the people, in exchange for an incorruptible God, an image made like to corruptible man. Wherefore God, in punishment for their sins, thus turning his truth into a lie, suffered even their mysteries, which they had erected for a school of virtue, to degenerate into an odious sink of vice and immorality; giving them up unto all uncleanness and vile affections.
They glorified him not - They did not give him that worship which his perfections required.
Neither were thankful - They manifested no gratitude for the blessings they received from his providence, but became vain in their imaginations, διαλογισμοις, in their reasonings. This certainly refers to the foolish manner in which even the wisest of their philosophers discoursed about the Divine nature, not excepting Socrates, Plato, or Seneca. Who can read their works without being struck with the vanity of their reasonings, as well as with the stupidity of their nonsense, when speaking about God? I might crowd my page with proofs of this; but it is not necessary to those who are acquainted with their writings, and to others it would not be useful. In short, their foolish, darkened minds sought God no where but in the place in which he is never to be found; viz. the vile, corrupted, and corrupting passions of their own hearts. As they did not discover him there, they scarcely sought him any where else.

Because that, when they knew God, they (e) glorified [him] not as God, neither were thankful; but became (f) vain in their imaginations, and their foolish heart was darkened.
(e) They did not honour him with that honour and service which was appropriate for his everlasting power and Godhead.
(f) As if he said, became so corrupt in themselves.

Because that when they knew God,.... Though they had such a knowledge of the being and perfections of God, yet
they glorified him not as God. They neither thought nor spoke honourably of him; nor did they ascribe those perfections to him, which belonged to him; they did not adhere to him as the one and only God, nor honour him as the Creator of all things out of nothing, and as the sole Governor of the universe; they did not glorify him by the internal exercise of fear of him, love to him, or trust in him, nor by any external worship suitable to his nature, and their own notions of him, Seneca is an instance of this, of whom Austin (f) says,
"that he worshipped what he found fault with, did what he reproved, and adored that which he blamed.''
Neither were thankful; neither for the knowledge of things they had, which they ascribed to themselves; nor for their mercies, which they imputed to second causes:
but became vain in their imaginations; the vanity or their minds was the spring and source of their evil conduct; which may design the wickedness of their hearts, and the imaginations thereof, which were evil, and that continually; the pride of their natures the carnality and weakness of their reasonings, and the whole system of their vain philosophy; and hence they ran into polytheism, or the worshipping of many gods:
and their foolish heart was darkened; where they thought their great wisdom lay: darkness is natural to the hearts and understandings of all men, which is increased by personal iniquity; Satan is concerned in improving it, and God sometimes gives up the hearts of persons to judicial blindness, which was the case of these men.
(f) De Civitate Dei, l. 6. c. 10.

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 Jeremiah 2:5).
in their imaginations--thoughts, notions, speculations, regarding God; compare Matthew 15:19; Luke 2:35; 1-Corinthians 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!

Because, knowing God - For the wiser heathens did know that there was one supreme God; yet from low and base considerations they conformed to the idolatry of the vulgar. They did not glorify him as God, neither were thankful - They neither thanked him for his benefits, nor glorified him for his divine perfection. But became vain - Like the idols they worshipped. In their reasonings - Various, uncertain, foolish. What a terrible instance have we of this in the writings of Lucretius! What vain reasonings, and how dark a heart, amidst so pompous professions of wisdom!