Evangelical Christian districts are literally impossible because that information isn't collected by the Census.
I'm sure that would change if the Supreme Court mandated such districts.
I suspect the religion clause of the 1st amendment would prevent that. There's a reason why the US Census doesn't ask religious affiliation when we ask so many other personal details about family, home and work.
I don't see how it would violate a strict reading of the First Amendment. The First Amendment specifically says "Congress shall make no law respecting an establishment of religion" but asking about religion would not require Congress to pass a law; it can be done entirely through internal Department of Commerce processes.
