Because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened.
In the early chapters of Romans, Paul shows the sinfulness of all men, gentiles and Jews. This verse follows the powerful statement that “His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse” (verse 20). God’s creation itself is proof that God exists, so even those without any biblical background are without excuse.
But the gentiles of Paul’s day put God out of their minds and didn’t give Him glory or show Him thanks. Many people today follow this same poor example. Ingratitude is a hallmark of human nature and especially of this end-time perilous age (as Paul mentions in 2 Timothy 3:2; see our article “Unthankfulness: A Sign of Perilous Times).”
Psalm 100, “a Psalm of Thanksgiving,” provides the cure for this common ailment. “Know that the LORD, He is God; it is He who has made us, and not we ourselves. … Enter into His gates with thanksgiving, and into His courts with praise. Be thankful to Him, and bless His name” (Psalm 100:3-4).