What is Forbidden in the First Commandment?

The first commandment forbids the denying, or not worshipping and glorifying, the true God as God, and our God; and the giving of the worship and glory to any other, which is do to him alone.

Psalms 14:1 (NASB)
The fool has said in his heart, “There is no God.”

Romans 1:20-21 (NASB)
For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse. For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened.

Psalms 81:11 (NASB)
“But My people did not listen to My voice,
And Israel did not obey Me.”

Romans 1:25 (NASB)
For they exchanged the truth of God for a lie, and worshiped and served the creature rather than the Creator, who is blessed forever. Amen.