I, too, am curious as to why so many people think “religious folk” would react any differently than anyone else. I know of no mainstream religious teaching that says “We are alone in the universe”. I’m sure one could find splinter groups who believe that, but one could find splinter groups who believe most anything.
I have heard arguments from ultra-conservative fundamentalist Christians that there cannot be extra-terrestrial life because:
-The creation story in Genesis does not mention them.
-Jesus came to Earth to saves us form our sins, and since aliens could not be similarly saved, they must not exist. God would not have created intelligent life that could not be saved.
Note that these are generally the same people who are invested in the belief that creation took literally 6 days and that the Earth is only 6000 years old. Young Earth Creationists, in other words.
I’ve heard other less conservative groups talk about the universe as being “a vast wonder filled with God’s creation”, and those folks sounded open to the idea of aliens. In their minds, Genesis is less literal and there is no reason God could not have sent a savior to any planet. The idea being that the Bible is a record pertaining to our planet and aliens may well have their own Bible and “Jesus”. I also heard one person express the opinion that we would be doing God’s work by trying to find and contact aliens, so that we could share the gospel with them.
So, certainly many religious folks have no problem with the idea of alien life existing somewhere in the cosmos. . . But there are those who do object to the concept based on their own religious beliefs.
Since I’m already 100% certain there is life on more places than just Earth, it wouldn’t change a thing for me.
There is a pretty mainstream idea that the creation on Earth was a special event. Man was made in God’s image, and all of Earth’s history and future are a pre-ordained plan to create and then redeem man for an everlasting partnership in Heaven. Pretty much every Christian will agree with that overall statement, even Protestants and Catholics.
The existence of alien life doesn’t exactly contradict this idea itself, but it might contradict things people have said about it. i.e. that the lack of alien life is proof that God’s plan is only concerned with the salvation of mankind. Having made their own statements and explanations based on one possible interpretation, people have been known to refuse to change their interpretation.
For example, I’ve never understood why Catholics ever made such a fuss about trans-substantiation in communion. Nothing in the Bible ever said the bread and wine were more than a symbolic representation. And yet, at points in history, questioning trans-substantiation could get you killed.
The OP’s hypothetical does not specify intelligent life.
Does this certainty derive from evidence, or is it faith-based?
I agree that it would be like learning about Darwin’s theories. Some would deny it for whatever reasons, some would find it interesting to add a new chapter to the textbooks, and human understanding of the universe. I wouldn’t be that surprised if we found microbes somewhere else in our solar system some day. It would be exciting, but not shocking.
Whatever is preventing us from ever, ever, ever going out there terrifies me. 
Does indirect evidence count? We’ve discovered dozens (hundreds?) of worlds where life, as we know it, could exist…and that’s just in our own stellar neighborhood. Expand that to billions of entire galaxies, and the likelihood of our own form of life being absolutely unique seems very, very hard to defend.
It takes a leap of faith to assume this: that the conditions on earth are not replicated anywhere else in the cosmos. At very least, this premise needs some evidential justification which is not at all visible, whereas the existence of planets fit for life – some more fit for life than earth is – is an observed fact.
If just life were found, even macroscopic life, I don’t think it would affect anyone’s beliefs very much. Personally I’d be quite surprised if there isn’t life on lots of planets.
Intelligent life is something different. I’d be quite excited to learn there some other intelligent species in the universe. But I’m not positive that intelligence is an obvious outcome of evolution, or how long intelligent species survive. It isn’t difficult to imagine that mankind will wipe itself out or at least render itself non-technological within 100 years. That would mean we were detectible for a tiny fraction of Earth’s existence. Knowing others “made” it might be promising.
if alien life were found on a planet hundreds of lightyears away there would probably come lots of theories and lots of new man made technology.
I think theories would be based on where the aliens were in their course of evolution and what their intentions might be. What we could see from them would be possibly hundreds, thousands or even millions of years old. it would all depend on how we discovered them or what we knew about them. It could be ancient radio signals from a lost civilization or it could be radio signals from a type 1 civilization. Its possible that we would prepare ourselves for some kind of interstellar defense. There would definitely be a better effort to find more life outside earth. I think all areas of space exploration would take a front seat for a long time. I wouldn’t doubt that the tech fields would make massive leaps forward with all the government funding they would be getting. Colleges would be churning out lots of tech and cosmos related grads. And presumably this would be front page / state of the union speech worthy stuff for a long time.
I don’t know what the religious might believe. I assume alien life, especially intelligent alien life would fly in the face of western religion. But I’m sure the religious would find some way to let their faith convince them that it is all part of they’re deity’s plan.
I get that, but you are both talking about a relatively small subset of Christians and a tiny subset of “religious folk” - those I referred to in my post as “splinter groups”. The first few posts in the thread made it seem that all people who have religious beliefs would have to change those beliefs should extraterrestrial life be found. It just ain’t so!
In David Brin’s novel “Existence”, we find incontrovertible proof of the existence of life beyond our own planet, in orbit around out own planet! Part of the novel discusses society’s reaction to the discovery; other parts describe the search for other “existence” artifacts on and around Earth; other parts of the story involve negotiations with the beings apparently inside the artifacts.
Like the knowledge that the Earth isn’t the center of the universe, or that the universe is way huge with lots and lots of other planets in it, the fact of life elsewhere that we’ll never have any contact with doesn’t really have any religious or practical significance for us. But some people will object to it on religious grounds, because it shakes up what they assumed about Life, The Universe, and Everything, and threatens their special uniqueness.
I’m an average person, and I happen to believe with 100% certainty that there is life out there. I came to that realization many years ago, and it hasn’t had any measurable effect on my life.