Would agree that any law against Sharia Law would be most likely unconstitutional. A law against those parts of Sharia Law, that violates our own Constitution, would probably pass. Much like Freedom of Speech does not include the call for the violent overthrow of the government. Though calling for the peaceful and democratic overthrow would probably pass muster.
As to the Second Amendment, murder, unless authorized by the government, they do have a monopoly on violence, is against the law, Christian Soldier or not.