NET

21 For although they knew God, they did not glorify him as God or give him thanks, but they became futile in their thoughts and their senseless hearts were darkened.

Darby

21 Because^D7972, knowing God, they glorified [him] not as God, neither were thankful; but fell into folly in their thoughts^D7973, and their heart without understanding was darkened:

Darby Bible Notes

Because - This is the second reason. the first is ver. 19. thoughts - The 'inward reasonings of the mind,' as Luke 9.46. The word 'thoughts' in English conveys this best.

KJV

21 Because that, when they knew God, they glorified [him] not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.

Strong's

21 Because ^G1360 that, when they knew^G1097 God^G2316, they glorified^G1392 him not^G3756 as^G5613 God^G2316, neither^G2228 were thankful^G2168; but^G235 became vain^G3154 in^G1722 their^G846 imaginations^G1261, and^G2532 their^G846 foolish^G801 heart^G2588 was darkened^G4654.