John Calvin Commentary Romans 1:21

"because that, knowing God, they glorified him not as God, neither gave thanks; but became vain in their reasonings, and their senseless heart was darkened." — Romans 1:21 (ASV)

For when they knew God, etc. He plainly testifies here that God has presented to the minds of all people the means of knowing Him, having so manifested Himself by His works, that they must necessarily see what they, on their own, do not seek to know—that there is some God; for the world does not exist by chance, nor could it have come from itself. But we must always bear in mind the degree of knowledge in which they remained; and this becomes clear 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 sustains 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 moved to preserve them—His justice, because in His government He punishes the guilty and defends the innocent—His mercy, because He bears with such great forbearance the perversity of men—and His truth, because He is unchangeable.

Therefore, the one who has a correct understanding of God ought to give Him the praise due to His eternity, wisdom, goodness, and justice. Since people have not recognized these attributes in God, but have dreamed 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, for there is no one who is not indebted to Him for countless benefits. Indeed, even for this reason alone—because He has been pleased to reveal Himself to us—He has abundantly made us indebted to Him. But they became vain, 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 so their foolish mind, being involved in darkness, could understand nothing correctly 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 to maturity.