The US, for most of our history, had a unique view of rights. Rights were not granted, they were inherent based on your existence. You don't get some rights if you are of a certain class, other rights if you are of a certain faith, etc. Of course we've had failings on this, but it has been our general foundation. Your rights are based on you as an individual regardless of your beliefs or class. The only way those rights are stripped is if you specifically do something to relinquish them (ie commit a crime). Having a belief is not a crime, no matter how henious that belief may be. It is the choice on how you act on that belief that is where the risk of losing rights may come into play.
Just believing in Mohammad, or hell, believing in Sharia law, is not justification for a citizen losing their rights unless they act upon those beliefs in a way that is criminal. When we give the power to the government to start stripping rights based on nothing more than a thought or a belief, you give up the very foundation of our country. That would be more than a slippery slope, that would be legal precident that would, of course, open the door to more and more.
Remember, who is it now the government classifies as domestic terrorists and the biggest threat? Hint, they don't pray five times a day.