Jamieson, Fausset and Brown on Romans 1:21

21. 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 Jr 2:5).

in their imaginations--thoughts, notions, speculations, regarding God; compare Mt 15:19; Lu 2:35; 1Co 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!