What part is unconstitutional?
I they were born in the USA, or went through the naturalization process, you can't deport them based on their religion (or, even, lack thereof).
They would have the same 1st amendment protection you and I have.
The best you could hope for, is what Carter did (of all people) and what Cruz proposed, and what Trump has hinted towards lately.
Ban people coming in from countries with known hostilities towards the USA.
Can't ban them if they are already citizens.
If you could, every flag burner would kicked out.
It is unconstitutional to do so.
Imagine if the next President or congress member decided that we had to get rid of all the Christians, or all the Jews?