Shorter Catechism Q. 47. What is forbidden in the first commandment? A. The first commandment forbiddeth the denying, or not worshiping and glorifying the true God as God, and our God; and the giving of that worship and glory to any other, which is due to him alone.
Commentary: The first commandment forbids atheism There are two types. The first is philosophical atheism. Paul says that all men know God, but because they suppress the truth about God in unrighteousness they believe and argue that there is no God. They deny his existence. The second type is practical atheism. Some people profess faith in God, but they live as if there is no God. They don’t obey or worship him.
This commandment also forbids idolatry. We commit idolatry when we worship other (false) gods, that is, when we give them the worship and glory that is due to God alone. Sadly, we can and we do turn anything or anyone into a god. Covetousness, Paul says, is idolatry (Col. 3:5). To covet is to want something too much or to want something that you are not allowed to have. It is to make something supreme in your life. Hence, if you crave money or sex or the approval of your parents or power or pleasure to the point that you are willing to do whatever it takes to get it, then you have turned that person or thing into a god. Whatever we want more than pleasing God is a god to us, a god that we serve worship and glorify.
See Westminster Larger Catechism 105 for a lengthy list of the sins that are forbidden by this commandment.