There's an exchange between two characters in one of Shakespeare's plays. In it Glendower says to Hotspur, "I can call forth demons from the vasty deep." To which Hotspur replies, "So can I. So can any man. But will the come when you call them?"
President Trump can issue any proclamation he wants and ban every Muslim on the planet from entering the U.S. He still has to enforce it and he has to identify Muslims as they try and come in. They don't have bar-codes. They don't have their religion on their passports. They come from every country and every race and they may or may not be willing to abide by the ban. So how do you identify they to ensure not a single one enters the country?
While we do not have sufficient data to thoroughly vet these people in terms of their detailed activities, I believe we can determine with some degree of certainty whether they adhere to Islam.
For example, the woman who came in on the VISA to be Syed’s wife, not only indicated she was, but Pakistan had the data that she had been a member of the Red Mosque.
There are relatives that can be asked. There are acquaintances that can be asked. They can be observed. In many places there is basic birth data.
Such data is not enough to determine if they are radicalized perhaps (though in that woman’s case it most certainly should have been IMH).
Finally, if we find that that is too difficult, we can do exactly what others have done...no Iranians, No Syrians, etc. from nations that are by far and away predominantly Islamic. I have a feeling this data point will be the initial deciding one.