Romans 1:21

“Because that when they knew God, etc..] 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 f6 says, “that he worshipped what he found fault with,…”

“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 Mat 15:19; Luk 2:35; Co1 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 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…”

“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! Romans 1:23”

“1:21 To know God in Scripture usually means to have an intimate, saving relationship with him (see 2 Cor 5:16; Gal 4:9; Phil 3:8, 10). Here, however, they knew God means that people knew about God. All people have some understanding of God through creation, yet they do not do what is right based on that knowledge. Rather than learn more about God, they worship gods of their own making.”

“when: Rom 1:19, Rom 1:28; Joh 3:19 they glorified: Rom 15:9; Psa 50:23, Psa 86:9; Hos 2:8; Hab 1:15, Hab 1:16; Luk 17:15-18; Ti2 3:2; Rev 14:7, Rev 15:4 but became: Gen 6:5, Gen 8:21; Kg2 17:15; Psa 81:12; Ecc 7:29; Isa 44:9-20; Jer 2:5; Jer 10:3-8, Jer 10:14, Jer 10:15, Jer 16:19; Eph 4:17, Eph 4:18; Pe1 1:18 their foolish: Rom 11:10; Deu 28:29; Isa 60:2; Act 26:18; Pe1 2:9 Romans 1:22”