Let's try a thought experiment. Suppose the poobahs of the Third Reich had declared Nazism a religion? Announced its leaders were bishop, cardinal and pope equivalents.
Would that have made Nazis untouchable under U.S. law? Could we not have banned them or their activities and propaganda because of the protections offered by the establishment clause? If this hypothetical seems ludicrous, explain how terroristic political Islam is not the moral equivalent of a "Hitler as Pope?" I'd argue for equivalency.
I saw a great analogy the other day, I wish I could remember the source to cite it. The analogy was this: suppose I have a bowl of M&Ms and tell you that a random 10% are poisoned. I ask you how many would you like to eat? Your wise answer: none.
Then I tell you that surveys show 10% of Muslim immigrants reject our values and wish us ill, how many should we admit? Your rational answer is the same: none. In both cases, the upside is far outweighed by the downside.
Neither M&Ms nor Muslim immigrants are an essential ingredient of a full life.