There has been an awful lot of ink spilled over many, many a decade on the question of what the discovery of an advanced extraterrestrial civilization would mean for terrestrial religions, especially the Big Three - Judaism, Christianity and Islam. That's mainly because the Bible (and associated texts) make no mention of ET, and thus human beings are the one and only apex of God's creations. Discovery of ET would throw that alleged apex into more than just a bit of theological confusion.
However, theologians tend to make light of this and suggest along the lines that there's an awful lot of real estate out there, and since God can do whatever He damn well pleases, well maybe He populated some of that real estate with one or more extraterrestrial societies. I mean the Bible doesn't mention Antarctica or penguins. The discovery of both didn't upset the religious applecart, so why are aliens relevant to vie for Biblical space and commentary and why should they, if they exist, upset the religious applecart?
In short, one answer boils down to, is everything out there in parallel with everything down here, at least as far as the big three monotheistic faiths go? Do all planets with intelligent aliens have extraterrestrial Adams and Eves that muck up their alien Gardens of Eden and get the boot? Do all alien civilizations have sin, a flood, a Chosen People, a Promised Land, etc?
Now apparently the biggest of the big theological question is, assuming the existence of ET civilizations, is whether or not Jesus (assuming the reality of a Jesus of course - not a given) visited these worlds and got subjected to the ET equivalent of The Cross. Did Jesus have to hitch a ride on interstellar spaceships in order to get to all those other sinful other-worlds, assuming those other-worlds are sinful other-worlds? Methinks the questions are as similar to how many angels dance on the head of a pin!
IMHO, the odds that our religious histories in broad-brush form would happen on each and every other-world housing an extraterrestrial civilization is so remote as to not be worthy of even two seconds of pondering.
Okay, so if UFOs land on the White House lawn tomorrow, or radio astronomers detect obviously artificial radio signals from an extra-solar other-world planet that's home to ET, big deal. Church attendance will probably not alter greatly, at least after the initial shock. If those of the monotheistic faiths embrace all of humanity as equals, then it's not a huge step upwards to embracing extra-solar 'humanity' - ET - as equals as well.
But, and this is a very real but, what if our advanced aliens are not just technologically advanced aliens, but theologically advanced aliens, who in fact have a theology that bears no similarity with any terrestrial theology! Then what? Might ET take a leaf out of our religious histories and violently preach their version of hell, fire and brimstone to us? What leaf you ask?
The basic reality is that members of the trilogy of major monotheistic religions (and lots of minor ones as well) have in the past wished, and continue to wish, to impose their beliefs by any means fair or foul (usually foul) on anyone and everyone else. If fact, all too often those wishes were turned into reality.
If one had to list all of the atrocities inflicted on various cultures by Christian missionaries, including the abduction and indoctrination of young children, well let's just say comparisons with the Nazi Third Reich regime wouldn't be all that inappropriate. From across Africa to the Pacific Islands and points beyond, it was the Christian duty of the faithful to force-feed if necessary their religious doctrines to all those thus far spared monotheism fanaticism. And it wasn't just a matter of polytheistic to monotheistic conversion, it was the absolute and total destruction of anything and everything part and parcel of their 'pagan' traditional beliefs that had to be eliminated, so much so that most of the culture, say of the Aztecs and the Incas, have now been lost forever - thanks due to God, or rather His ever faithful representatives.
Then throw in the Inquisition, the Crusades, and all manner of Holy Wars and God's Old Testament reign of terror has been taken to heart by the faithful whose duty is to see that it is 'to be continued' and on, and on, and on it goes. Your option: be a living Christian; or a dead pagan. Well there's an exception to that - the last of the Inca emperors was given this option: a relatively quick and easy death as a born-again Christian, or a very slow and very painful death as a pagan. Needless to say Christianity won out yet again. Belief in God can be very persuasive when you're faced with being burned at the stake as an alternative.
Albeit more civilized today, the indoctrination goes on. It might be religious fanatics picketing in front of abortion clinics or forcing public schools to delete Darwinism (Darwinian evolution) from their curricula and replace it with Creationism or Intelligent Design (you'd think that had been settled once and for all with the 1925 Scopes Trial). It often takes the form of all those televangelists knocking your socks off and all those religious billboard signs warning you of this, that and the next sinful thing. Then of course there are those ever pestering Bible-pushing Christians knocking at your door, eager beavers telling you how much God loves you, but in return for a donation He'll love you even more!
But take note, its God the singular, not gods the plural. I mean is it a God / Jesus Bible-thumper who bangs on your door or is it an Osiris / Odin / Quetzalcoatl / Zeus, etc. person who disturbs your peace and quiet, trying to convert you to the wisdom of polytheism? Did I hear you say God / Jesus? I thought so. Despite the fact that the Bible isn't a legal document like a search warrant or a summons, it nevertheless seems to give Bible-pushers carte blanch to do whatever they please, as long as the Bible tells them it's okay to do it, like chewing your ear off (not literally of course) with tales of hell, fire and brimstone and trying to scare the shit out of you into making a donation to the cause.
If there were any polytheistic cultures who tried to ram their gods down the throat of other cultures I'm not aware of them, which is not the same as other cultures assimilating the gods of another culture. As an example, there ultimately proved to be an amalgamation of ancient Greek and Egyptian deities. Ramming has been the 'divine right' and privilege of monotheistic cults and examples, including all of the very graphic details, would fill an encyclopedia. Would aliens perceive their having a 'divine right' to ram their theology down our throats? Yes, if our own history is an example. If God is on your side, you can do no wrong!
In the history of our terrestrial civilization, there have been lots and lots of refugees. Many are economic, escaping poverty by chasing that pot of gold at the end of the rainbow away from their homeland where it never rains (thus no rainbow). Some are political. Some are for racial / racism reasons. A fair share of all refugees, past and present, are religious refugees, an obvious example being the Pilgrims that migrated to the United States of America before there was such a name as the United States. So the issue of religious freedom, or freedom from having someone else's religion rammed down your throat, is not trivial.
When it comes to ET, the traditional Hollywood image, often reinforced by some scientists, is that when the aliens come calling, it will be with ray-guns blasting away (like in "The War of the Worlds"), unless they are sneaky like in "Village of the Damned" plus sequel "Children of the Damned", or "Invasion of the Body Snatchers" (several versions). Justification for depicting ET as nasty is often given based on terrestrial histories of invasions and conquests. A warfare scenario pessimist is hardly ever disappointed; a peace scenario optimist frequently is.
But let's look on the bright side. ET arrives and there's not a ray-gun in sight. It's the dawn of a new and enlightened era! But let's substitute their ray-gun replacing it for their extraterrestrial religious text, a text in which ET worships the Almighty Green Slime Supreme Being, Lord of All That Oozes (or in Hollywood fare, "The Blob").
Okay, so based on religious tolerances (or lack of same) as expressed within and by terrestrial societies, what can we expect from ET? When it comes to a religiously inclined and pushy ET, well that's hardly ever mentioned as a possible scenario, but ultimately it's really terrifying, in a potential sort of way. A fanatical religious ET vis-à-vis an invading ET, is perhaps a more likely ET and what we may really need to worry about is that they'll come in peace alright, but as fervent missionaries to spread the word that their version of a supreme being [the Almighty Green Slime Supreme Being, Lord of All That Oozes] is the only true version of a supreme being and that all of us terrestrial heathen, pagan infidels had better see the error of our untrue faiths and convert to their extraterrestrial theology quick-smart.
More likely as not ET won't be Jesus-like and certainly won't allow themselves to be nailed to a cross (or a higher-tech version); they will probably be more along the lines of the Conquistadors or perhaps akin the Inquisition, or even worse Koran-thumping Islamic extremist-types. After all, they have to be very, very strongly motivated to come out to our neck of the boonies, and they are going to want favorable results, or else. All hail the Almighty Great Green Slime Supreme Being, Lord of All That Oozes!
Now here's a very rough translation and commentary on the first few bars (it's also an opera) of ET's "Holy eBook of Slime and Ooze". It goes something like this:
"Once upon a time the Almighty Great Green Slime Supreme Being, Lord of All That Oozes, ejaculated and gave rise to the Holy Ooze and the Holy Ooze was without form and the Almighty Great Green Slime Supreme Being, Lord of All That Oozes felt that this was how it should be and everything was Almighty fine. And that ended the First Great Cycle of Cycles.
On the Second Great Cycle of Cycles, Ms. Almighty Great Green Slime Supreme Being, Lord of All That Oozes took her less than better half partner, the Almighty Great Green Slime Supreme Being, Lord of All That Oozes to task for creating a Holy Oozy Mess and told her less than better half, the Almighty Great Green Slime Supreme Being, Lord of All That Oozes to clean it up, or else! And so it came to pass that the Almighty Great Green Slime Supreme Being, Lord of All That Oozes swept the Holy Ooze all under the Heavenly Cosmic Carpet.
But the Almighty Great Green Slime Supreme Being, Lord of All That Oozes wasn't through with His creation, and under cover of the Cosmic Night, He played with His toy biochemistry set and infused animation into the Holy Ooze that had been swept under the Heavenly Cosmic Carpet. And that was the Third of the Great Cycles.
The animated Holy Ooze multiplied in an Oozy sort of way and expanded outwards, ever outwards from under the Heavenly Cosmic Carpet and the Almighty Great Green Slime Supreme Being, Lord of All That Oozes was Almighty pleased and totally up Himself with His skills. And thus was concluded the Fourth Great Cycle of Cycles.
On the Fifth Great Cycle of Cycles, Ms. Almighty Great Green Slime Supreme Being, Lord of All That Oozes, totally fed up with her less than better half, packed her Heavenly Duffle Bags and left the Almighty Great Green Slime Supreme Being, Lord of All That Oozes for even greener pastures. She moved in with the step-brother of the Almighty Great Green Slime Supreme Being, Lord of All That Oozes. His name was the Greater Greener Slime Being, who, unfortunately had fallen from Grace [mutiny I believe] and had been stripped of his Almighty Supreme title and status - such are the fates in family disputes and Wars in Heaven between rivals for power."
And it goes on and on and on from there, over ten eBook volumes worth that kind of make the Bible read like a short story. But in a bit of a twist, a role reversal, the underdog, the Greater Greener Slime Being ultimately triumphs and comes out on top to become the new Almighty Greater Greener Slime Supreme Being. Well after all the Almighty Great Green Slime Supreme Being, Lord of All That Oozes really was a bit of a twit.
The former Greater Greener Slime Being and the former Almighty Great Green Slime Supreme Being, Lord of All That Oozes' Ex - His more than better half - together, it is prophesized, will have a child who ultimately will become the role model for, obviously, "The Son of the Blob", who is to grow so blobby that His massive gravity will prove enough to cause the Heavenly Cosmic Carpet and all that it contains (life, the universe and everything) to roll up in on itself and turn into a singularity. It will be the End of Days, the Cycles of Cycles - the Big Crunch will have arrived! If I recall correctly, that Last Great Cycle is numbered somewhere over the million mark of all things Cyclic. Amen.
Although you'd hope that advanced alien beings would have long since out-evolved such religious nonsense, that's not a given. That missionary scenario is even more frightening than them coming here with their ray-guns blasting away IMHO. So if those extraterrestrial evangelists come knocking at your door, with tales about the Holy Ooze, or the Almighty Greater Greener Slime Supreme Being, be afraid, be very afraid!
Science librarian; retired.