Q. 52: What is forbidden in the first commandment?
A. The first commandment forbids the denying, or not worshiping and glorifying the true God, as God and our God, and the giving that worship and glory to any other, which is due unto Him alone.
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. Romans 1:21