Catholics aren’t opposed to it.

“God became man in Jesus in order to save us. So if there are also other intelligent beings, it’s not a given that they need redemption. They might have remained in full friendship with their creator,” he said.

Father Funes went on to say that Christ’s incarnation and sacrifice was a unique and unrepeatable event. But he said he was sure that, if needed, God’s mercy would be offered to aliens, as it was to humans.

I don’t think this is like official doctrine, but it seems to be the most common attitude I’m finding.

As far as Protestants: the flat earth types might be against aliens because that goes against their everything. I’m really only talking your KJV-onliest or freaky Pentecostal teetering into cult types that would think aliens were literal demons.

But I think most fundies would probably be fine with aliens if they were allowed to proselytize to them, and most mainline Christian churches would be fine with aliens existing.