A.T. Robertson 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)

Because that (διοτ). As in verse 19.Knowing God (γνοντες τον θεον). Second aorist active participle of γινωσκω, to know by personal experience. Definite statement that originally men had some knowledge of God. No people, however degraded, have yet been found without some yearning after a god, a seeking to find the true God and get back to him as Paul said in Athens (Acts 17:27).

Glorified not as God (ουχ ως θεον εδοξασαν). They knew more than they did. This is the reason for the condemnation of the heathen (2:12-16), the failure to do what they know.

Their senseless heart (η ασυνετος αυτων καρδια). Καρδια is the most comprehensive term for all our faculties whether feeling (Romans 9:2), will (1 Corinthians 4:5), intellect (Romans 10:6). It may be the home of the Holy Spirit (Romans 5:5) or of evil desires (1:24). See Mr 7:21f. for list of vices that come "out of the heart." Ασυνετος is a verbal adjective from συνιημ, to put together, and α privative, unintelligent, not able to put together the manifest evidence about God (verse 20). So darkness settled down on their hearts (εσκοτισθη, first aorist ingressive passive of σκοτιζω, to darken).