There is a word too many choose to ignore: SEDITION.
Islam plainly, and openly, declares that it supersedes all government authority and recognizes no laws other than its own. It also seeks the overthrow of all authority other than Islam and the submission of all to its laws.
If that isn't seditious I don't know what is.
It's the term 'religion' that Islam is erroneously protected by in the Constitution. IT may call itself that-but we do not have to agree-especially considering what it is on paper-vs- what it really is.
Refuse to recognize it as a 'religion' and it becomes just another cult of destructive ideas and people.
The Constitution is being threatened BY Islam alright, but not in making Islam illegal. It is threatened by allowing Islam- which by its doctrine CANNOT and WILL NOT be subject to the Constitution- to exist in this country as anything other than a seditious,violent,dangerous cult.
How long its been around doesn't matter. Age should not protect it, nor should semantics.
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.