That is my view—”they” have always been here and the “aliens from outer space” is disinformation—from them!
I'm about to get laughed at, but...
Demons. :) (Fallen angels, celestial entities, whatever you prefer).
I mean, if you're the type of person who takes the Bible literally (when it's meant to be literal), then acknowledging the reality of demons isn't a leap. They were cast down and occupy our realm... they're technically extraterrestrial, and arguably they operate with advanced knowledge from what we mere mortals possess.
Okay... stop laughing, but... there it is.
Add to your reading list: The Three Body Problem, The Dark Forest and, Deaths End - a trilogy by Cixin Liu.
The Dark Forest answers Fermi’s conundrum - if there are aliens, where are they?
I’ve read most of the others.
I have an idea about what that major ‘religion’ might look like.