Extraterrestrial beings would regard Earth's terrestrial, man-made religions the same way we homo-sapiens regard the hootings and hollerings of wild chimpanzees in the jungle.
Interesting? Absolutely. And should be archived and preserved for academic study. Useful? Again, absolutely. Such screeching alerts other members of the tribe to look out for potential predators. Or where to find the nearest fruit.
But insightful into the actual workings of the Universe? Not at all. And not even close.
So you're suggesting alien evangelists will have more luck spreading their faiths to us? That is a real possibility, too. Easy for a charismatic alien displaying advanced tech to seem like a guru worth following.
I would not rule out conversion the other way, though, or syncretism to form new alien friendly versions of Christianity and Islam and Buddhism, at least. Much like Mormonism is an attempt to update Christianity for the Americas, a new space religion incorporating some alien space God as being the same being actually as God the Father might be a popular new version of Christianity.
So here's a notion that sounds wild but when you think about it is not only plausible but it makes sense: "Alien Christianity" would not only be a possible future thing in this scenario, it's entirely possible that it (along with "Alien Islam/Buddhism/etc.") already exists!
Any society technologically advanced to be able to make contact with us is clearly also one that would have technology to monitor and study our world well beyond ours. This wouldn't be some sci-fi movie where they land and then have to learn all about our planet and society...they obviously would have to have ways to already observe and learn this, think of how we use satellites for observation now, who knows if they have dumped some undetectable with our current technology nanomachine satellites into our atmosphere and have been monitoring our planet via them for decades if not centuries. So by the time any contact happened, I think it's safe to say that the aliens would have knowledge of the planet's largest religion. And furthermore you know how people from advanced developed cultures on Earth love to appropriate things that they find unique and exotic from other cultures, including religions? (See white Buddhists, neopagans, etc.) It's not really a stretch to think that some aliens in that civilization would actually find Christianity so fascinating they'd start practicing it themselves (or some syncretic spin on it with their own society's history, etc.) And considering the time frame it's possible it's been around centuries as well, so maybe even had the chance to develop into a real notable religion on that world instead of just the equivalent of Wiccans today or whatever.
In fact, it's not unfeasible first contact would be made by these "alien Christians", after all if given the opportunity why wouldn't they want to visit the Holy Land?
