Question: "If aliens were proved to exist, how would that discovery impact the Christian faith?"
Answer: Believe it or not, theologians, astronomers, and science fiction fans and writers have contemplated this question long and hard. The debates have narrowed down where the problems would arise, but the variables prevent a definitive answer.
The most commonly discussed issue is that of the identity and work of Jesus. God sent His only begotten Son, God incarnate, to save mankind and redeem creation. Does that redemption include life on other planets? Or would God have manifested Himself on those other planets, as well (in the manner of Aslan in Narnia)? Does "only begotten" mean "only physical representation"? Or is it more limited, referring only to the human species?
Another consideration: would an otherworldly, sentient, advanced life form sin and need redemption in the same way we do? Human life is in the blood (Leviticus 17:11). Where is the life of these hypothetical aliens? And what would have to be sacrificed to save them? Could the human blood of Jesus save silicon-based glass creatures whose sin was melting and reforming themselves into unnatural shapes?
Another topic of discussion is what it means to be made in the "image of God." Since God has no physical body, we take this to mean a reflection of God’s non-physical aspects—rationality, morality, and sociability. Would aliens embody the same characteristics? We have no way of knowing.
One issue rarely broached is the impact of Young Earth Creationism on the discovery of alien life. It is conceivable, if highly unlikely, that the geological pyrotechnics that took place during the global flood could have spewed a bacteria- or lichen-tainted stone all the way to Mars where it found shelter in a misty canyon. But any life form more complicated or farther out would be much harder to harmonize with a literal reading of Genesis 1. Could demons have taken trees and shrubs and rodents and bugs to another planet with an environment similar to Earth’s? Possibly. But without the Spirit's blessing of life, it's unlikely any of it would have survived. Parallel creations? Maybe. The Bible does not mention parallel creations, but, then again, there's no reason why God would have to tell us about them.
If a more advanced race is found, it would be interesting to see if they had stories about creation, sin, redemption, and an omnipotent, all-seeing Deity. If so, the Bible would have even more external validation. If not, then we have another mission field.
Considering what we know about space and life and the world as God sees it, we already have an explanation for so-called alien activity on Earth. Reports of “close encounters” describe the ethereal, transient, deceptive, and malevolent. Accounts also record that encounters with supposed aliens can be stopped by a real, authentic call to Jesus. Everything points to the activity of demons, not extraterrestrials.
The discovery of alien life would have no effect on genuine Christianity. The Bible stands as written, and there is nothing in it that would absolutely contradict the existence of extraterrestrials. If aliens exist, they exist within God's framework for creation. God cannot be hurt by truth, and those who walk by faith love the truth. John 16:13 assures us that the Spirit will guide us to all truth. Nothing that happens in this universe is a surprise to God. If aliens exist, God will let us know what to do with that information when the time comes.