What is forbidden in the first commandment?

Answer

The first commandment forbiddeth the denying (), or not worshipping and glorifying the true God (), as God and our God (), and the giving of that worship and glory to any other, which is due unto him alone ().

Read the 4 proof texts (KJV)

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 1:21: 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 81:10-11: ¹⁰I [am] the LORD thy God, which brought thee out of the land of Egypt: open thy mouth wide, and I will fill it. ¹¹But my people would not hearken to my voice; and Israel would none of me.
Romans 1:25-26: ²⁵Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen. ²⁶For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature: