Because that, knowing God, they glorified him not as God, neither were thankful (rather, gave thanks); but became vain in their imaginations ( διαλογισμοῖς, elsewhere more correctly rendered "thoughts" or "reasonings;" cf. 1 Corinthians 3:20, "The Lord knoweth the thoughts of the wise, that they are vain"— μάταιοι, as here, ἐματαιώθησαν), and their foolish heart was darkened.
Professing themselves to be wise, they Became fools, and changed the glory of the incorruptible God into the similitude (literally, in similitude; cf. Psalms 106:20, whence idea and words are taken) of an image of corruptible man, and of birds, and fourfooted beasts, and creeping things. The expression, γνόντες τὸν θεὸν, refers to what has been said of τὸ γνωστὸν τοῦ θεοῦ, having been "manifest in them." It implies actual knowledge, not mere capacity of knowledge. Mankind is regarded as having lost a truer perception of God once possessed, idolatry being a sign of culpable degradation of the human race—not, as some would have us now believe, a stage in man's emergence from brutality. Scripture ever represents the human race as having fallen and become degraded; not as having risen gradually to any intelligent conceptions of God at all. And it may well be asked whether modern anthropological science has really discovered anything to discredit the scriptural view of the original condition and capacity of man. The view here presented is that obfuscation of the understanding ( σύνεσις) ensued from refusal to glorify and give thanks to known Deity. "Gratias assere debemns ob beneficia; glorificare ob ipsas virtutes divinas" (Bengel). Hence came ματαιότης, a word, with its correlatives, constantly used with reference to idolatry; cf. Acts 14:15; 1 Corinthians 3:20; Ephesians 4:17; 1 Peter 1:18; also in the Old Testament, 1 Kings 16:26 ( ἐν τοῖς ματαίοις ἐπορεύαὐτῶν, LXX.), 2 Kings 17:15 ( θησαν ὀπίσω τῶν μαρταίων, LXX.); Jeremiah 2:5; Jonah 2:8 ( φυλασσάμενοι μάταια καὶ ψευδῆ). Two forms of idolatry—both involving unworthy conceptions of the Divine Being—are alluded to, suggested, we may suppose, by the anthropomorphism of the Greeks and the creature-worship of Egypt, which were the two notable and representative developments of heathen religion. The expression, φάσκοντες εἷναι σοφοὶ, with the previous ἐν τοῖς διαλογισμαοῖς, have led some to suppose in this whole passage a special reference to the schools of philosophy. But this is not so. The degradation spoken of was long anterior to them, nor is this charge, as formulated, applicable to them. The idea is, generally, that boasted human intellect has not preserved men from folly; not even "the wisdom of the Egyptians," or the intellectual culture of the Greeks (cf. 1 Corinthians 1:19, etc.; 1 Corinthians 3:19, etc.).