This verse was a little unclear in the old King James so I have retranslated it to give the key points their correct meaning and emphasis:
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.
To:
Because knowing God, they did not honour him as God, neither gave thanks; but became worthless in their reasonings, and their foolish heart became darkened.