As soon as the law can prove what people believe. See, that's the problem. A law prohibiting DOING something has a chance of being enforced. A law prohibiting BELIEVING IN something is pure nonsense until someone invents a telepathy machine.
And making a law that can't actually be enforced (they can't prove that someone violated a law that prohibits belief) is just asking people to hold all laws in contempt, because the people who write such laws are idiots. (Or maybe the people they're making the laws for are even more idiotic, so they won't notice how stupid the law is. I haven't polled them, so I can't know.)