When we discover intelligent life in the universe we will have to face certain facts. The first would be that the appearance of an intelligent humankind was not a special act of creation by god(s). If intelligent life can arise in a multitude of conditions it makes it less arguable that God needed to act specially in order to create humankind. Yet such actions are part of the creation stories of all major religions. The particulars of our religions would be reduced to local symbolism and relative truths that apply only in some parts of the Universe, to some species.
If one group of sentient beings on Planet A have to abide by a religious book that was sent their way by a divine messenger, and sentient beings on Planet B have a different book, in a different language, with completely different stories of creation, sin and redemption, then how do adherents on either planet claim that their religion is the ultimate truth? It can't be the ultimate truth. It is only localized, temporary, limited truth, which applied only to one species, for the duration of their existence. I am sure that just like all human religions, part of the lore includes masses culture-specific, language-specific and time-specific texts that lose meaning over distance and time. If those two vastly different cultures meet on Planet C, then, by which ultimate truth would the future natives have to abide by? Clearly, new religions would be needed to cope with these complexities.
Imagine an intelligent race in the universe finding out what our religion says about us and our central position in the Creator's plan, and perhaps imagine trying to tell them that our religions are not born from our own imagination, pride and ego! Needless to say, we would embarrass ourselves.
The impact of extra-terrestrial life on theology is a lot more interesting than outsiders would presume. The existence of aliens therefore throws a long shadow of doubt over the entire Christian system of sin and redemption, Heaven and hell, and in particular, cause problems for the idea of an only-son-of-god saviour. Judaism, Christianity and Islam all have holy books that claim to be absolute truth, yet, their cosmology places humankind and the Earth in a special, unique position in God's plan especially with regards to the causes and solutions to suffering. When we find aliens and discover their religions, if they have them, our traditional religions will no longer make much sense.
In Christianity the Creator of the Universe has only one son, and that one son was born on Earth in the form of a Human, and died there in order to save humans (1 Peter 3:18). Such a sacrifice only occurred once - not once per planet, once per intelligent species, or even once per galaxy. There is no saviour for anyone who isn't on Earth. Aside from the Creator of the Universe having a human son, Humans were created in the image of God and the angels, who in Genesis 1:26-27 say "Let us make man in our image, in our likeness, and let them rule over the fish of the sea and the birds of the air, over the livestock, over all the earth, and over all the creatures that move along the ground". This special creation of humankind must distinguish us from any race of intelligent aliens, who obviously only merely evolved into greatness. And in Biblical eschatology, when evil and suffering is stopped in the whole of creation, where does God place his new paradise? On Earth, of course (Revelations 21)! That's quite an endorsement of our species, from the creator of a billion billion stars.
Is it amazing divine grace that us Humans are so blessed with the Creator's special and ultimate attention? Or is it an anachronism, written into our myths and religions during a time when Humanity did not yet perceive the vastness of the Universe? Given that Christianity fought major battles with astronomers about the Earth being the center of the world and with biologists about evolution, and lost both major battles, I would hazard a guess that when it comes to the uniqueness of the Human race in the Universe, Christianity also finds itself the wrong side of the divide between mythology and history.
The rationalist thinker Thomas Paine brought out some of these arguments in the 19th century:
“A plurality of worlds, at least as numerous as what we call stars, renders the Christian system of faith at once little and ridiculous. [It is a] strange conceit that the Almighty, who had millions of worlds equally dependent on his protection, should quit the care of all the rest, and come to die in our world, because, they say, one man and one woman had eaten an apple?"”
“In Christian theology Adam and Eve were punished for the original sin (of eating the apple); this punishment would be passed on from parent to child, and included not only suffering in general and death, but also many specifics, such as having a painful childbirth. Romans 5:12: "Therefore, just as sin entered the world through one man, and death through sin, and in this way death came to all men, because all sinned". Humankind was also expelled from the Garden of Eden at that point. Clearly, aliens on another planet are not subject to original sin, and, the punishments listed in the Bible may not even be applicable for the various possible life forms which exist in the Universe. Jesus was god's only son, crucified once here on earth, for us humans. Alien life makes a mess for the entire Christian idea of sin and salvation.”
“Making our Earth just one amongst many raised thorny questions about Christ's uniqueness. It was hard to reconcile multiple inhabited worlds with the fundamental tenet of Christianity, that God singled out the human race for special attention.”
“The existence of extra-terrestrial intelligence would have a profound impact on religion, shattering completely the traditional perspective of God's special relationship with man. The difficulties are particularly acute for Christianity, which postulates that Jesus Christ was God incarnate whose mission was to provide salvation for man on Earth. The prospect of a host of 'alien Christs' systematically visiting every inhabited planet in the physical form of the local creatures has a rather absurd aspect. Yet how otherwise are the aliens to be saved?”
Although Paul Davies talks of the humorous idea of thousands of Christ's resembling various aliens, being sent to all those planets by God, it contradicts the main messages of the Bible: that Jesus was God's only son and that the Crucifixion was a unique event specific to Jesus (1 Peter 3:18). So, there is no salvation for aliens. Also, Biblical eschatology has Zion descend on to Earth after Judgement (Revelations 10), with no more stars in the sky. Genesis 1:26 says that man is made in the 'image' of God and the angels, 'in our likeness'; so where would aliens come from? Are we to believe that intelligent life evolved throughout the universe but that our species in particular was created by the Creator, and that the Creator happens to look Human, along with the 'only' son of God who is also Human-shaped? To believe such things is known as anthrocentrism, a prideful and egotistical speciesism. Do aliens who live countless light-years away from Earth really have to know Jesus in order to be saved? But if you alter the Bible's stories to make them compatible with alien races on other planets, you also alter the central truths of Christianity; the religion you end up with after taking aliens into considering simply isn't Christianity any more.
These questions have few answers that Christians can accept and this problem has led some theologians down crazy routes. Some have postulated that all life in the Universe is descended from Adam and Eve; that aliens will also turn out to be Christians; or that because it is incompatible with the Bible, that there simply cannot be alien life elsewhere in the universe. We will see!
Some activist Christians have pushed for Christian messages to be sent out into space, and that our first priority should be to convert aliens. The horrendous consequences of this type of belief when Christians conquered the natives of Northern America and Australia are written into our history: They converted, or were killed. Hopefully such a violent and shocking confrontation will not also be part of humanity's future.
Links (some of the text above is extracted from these pages):
Mainstream theologians and intellectuals within the Abrahamic suite of religion have not given much time to discussions of the implications of alien life. However some have, and Erman McMullin warns his fellows that "religion which is unable to find a place for extraterrestrial persons in its view of the relations between God and the universe could find it increasingly difficult to command man's assent in times to come"5.
Some saviour religions actually cater for aliens. The Urantia movement holds that God has 700 000 sons incarnated on various worlds in the universe; Jesus is, of course, one of those divine sons6. The early scientist and astronomer Huygens argued that "the planets must be inhabited because otherwise God had made worlds for nothing"7. The religions that seem best placed to function in a Universe with more than species of intelligent, sentient beings are those that hold to universalist ideals; that is - the creator of the universe saves all living beings, and everyone goes to heaven. Other positions - that God punishes members of one species for doing 'evil' things, while punishing members of a different species for doing completely different things according to local circumstances, does not make sense if God embodies any set of absolute morality. In other words - deist, pantheist, universalist and non-religious forms of deism appear to be able to explain aliens much better than Abrahamic religions (Judaism, Christianity and Islam).
There are some non-traditional religions that are perfectly happy with the existence of alien intelligent life. These tend to be atheistic religions. For example many UFO cults are atheistic. The infamous Scientology is a sci-fi, self-help, anti-alien religion with beliefs that don't include any kind of theism (hence, it is also atheist). The Raelians in particular gained fame as the quintessential UFO-based religion. The actions of their "Clonaid" organisation, founded in 1997, was a veiled attempt at drawing people into their movement. It all started in The Bahamas and they knew the Bible very well, and informed us all that we'd mistranslated it quite badly. He had previously been a pop singer and racing-car journalist and it is somewhat a mystery as to how, inbetween 1973 and 1997, he managed to live a normal life with such a monumentous event behind him. Some people suspect he made the whole thing up.