Google 'Shinto'. The question is not whether or not Islam is a religion (it is, even if it is a death cult) - the question is whether or not its practitioners are afforded 1st Amendment protections. The litmus test is fealty to the Constitution. Others have mentioned sedition - we also have treason if linked to war making powers of foreign governments based on a hostile religion.
The US has already been through this with Japan. As the heat gets turned up against the muzzies, we can be certain that the SC will be going back to the 40s as guidance.
Well then. We have legal precedent.