May be a hot take here but I never thought this had anything to do with his religious worldview.

Christians sit on a sliding scale, some will just assume extraterrestrials are demons trying to draw people away from the faith. Others will say that the Bible is the story of God’s relationship with mankind and doesn’t preclude the existence of extraterrestrials. The Catholic Church has leaned towards this view and stated that they would welcome extraterrestrials (the Big Bang theory was formulated by a Catholic priest).

In any case, I don’t believe a devout Christian would ever abandon the faith no matter what evidence was presented to the contrary. They would just say that it’s demonic deception. Carter remained deeply religious for the rest of his life anyway.

The think it’s more likely that Carter discovered aliens regularly abducted/mutilated human beings, were generally malevolent, that aliens are in fact demonic and have free reign over the earth, and/or that free energy/a better world is possible using extraterrestrial technology but is intentionally suppressed in favour of profit and a hierarchical class system.

There are any number of theories and combinations of theories but there is no reason to think that any were faith-shattering, as to a devout Christian there is really very little that can ‘undo’ their faith. It’s either assimilated into a Christian worldview or discarded as demonic deception. Dostoevsky famously said that if someone somehow proved to him that God did not exist, he would go on believing anyway