Article Six of the United States Constitution
Article Six of the United States Constitution says that the Constitution, and any laws or treaties the United States might make, will be the most important laws in the United States.  It also says that the United States will pay debts from before the Constitution was written.  Finally, it says that members of the United States government have to swear that they will obey the Constitution.  However, they will never have to swear that they follow a certain religion.