not a single law that i am aware of in the US has made being a christian illegal. or prevented christians from worshipping their god.
of course, when their worship intersects the secular world, then they may run afoul of the law. for example, if a religion believes in human sacrifice, that would be illegal, because their "right" to human sacrifice is held to be less important than the right of people to live.
some religious morons seem to think that just because its part of their religion they have an unfettered right to perform those actions. duh