Q: What is forbidden in the first commandment?
A: The first commandment forbiddeth the denying, or not worshipping and glorifying the true God as God,2and our God; and the giving of that worship and glory to any other, which is due to him alone.

Psalm 14:1. The fool hath said in his heart, There is no God. They are corrupt, they have done abominable works, there is none that doeth good.
Romans 1:20-21. For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: 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.
Psalm 81:11. But my people would not hearken to my voice; and Israel would none of me.
Romans 1:25. Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen.

Westminster Project - See John Flavel on the First Commandment