State and federal law is the law for everyone in the country, no matter your religious or cultist beliefs....
Yes, but the law is not that simple. Suppose you and I enter into a as contract that states that the contract shall be governed under the laws of Italy. In nearly every state, the court will apply Italian law to resolve the dispute unless the law or the result violates an important policy of the state. If two ragheads agree by contract that disputes should be resolved by Sharia law, then it's their head to lose.