21 For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened.

Read Romans 1:21 Using Other Translations

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.
For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened.
Yes, they knew God, but they wouldn’t worship him as God or even give him thanks. And they began to think up foolish ideas of what God was like. As a result, their minds became dark and confused.