""I will implement strong ideological screening of all immigrants," Trump said. "... if you don't like our religion – which a lot of them don't..."
The US, of course, has no religion. There is no "our" religion; there is a plethora of religious beliefs and non-religious ones.
Whether Trump ignorantly doesn't know a religious test for entry would be struck down or knows it and doesn't care (because he never cares about the truth of what he's saying), it's useful pandering to his base of angry MAGA-minds. Most of them probably don't know, and many who do would like to see a religious test implemented anyway.
Of course even if implemented Goodhart's Law would apply: “when a measure becomes a target, it ceases to be a good measure.” So you'd have lots of people just pretending to care about Christianity.
But that's all the Trumpers want anyway. To many of them, Christianity is just a token of Americanism. You don't have to go to church or live a Christlike life or even know the basic tenets of the faith. It's like expressing your patriotism by wearing an American flag swimsuit that contravenes the flag code, at least in spirit.