If we found other sentient life, would it make you reject Christianity? As we find more and more intelligence in the life around us, I think it is especially important to look carefully at what really makes humans special.
Humanity separated by Intellect?
There has been a view that humans are separated from other animals by our intelligence, but this seems to be merely a different placement on the same scale. Octopi, dolphins, parrots and crows have all been marked out as smart animals and this seems to be becoming more apparent. Even anecdotally it seems that dogs are often roughly on par with human infants in cognition.
I think our blindness to animal sentience originates from a misunderstanding of human identity in Christianity. This misconception has then spread throughout broader society and is slowly being undone.
By Consciousness?
Perhaps a more viable, but less observable, distinction might be about consciousness. We might claim that humans have consciousness, while animals do not. However, how confident could we be in such a claim? Confirming that other beings are conscious may not even be possible (there are many philosophical issues that make this difficult). I trust that other people have consciousness, and it doesn’t feel like a massive jump to think that animals might also have consciousness of some kind.
I think the distinction between humans and other animals is found in characteristics that have been given to us in addition to our mundane traits; rather than what is inherent, that which has been endowed.
By Dominion
In Genesis, God gives humanity a privileged position as rulers over his creation, at least in the mundane physical world.
Then God said, “Let us make man in our image, after our likeness. And let them have dominion over the fish of the sea and over the birds of the heavens and over the livestock and over all the earth and over every creeping thing that creeps on the earth.”
So God created man in his own image,
in the image of God he created him;
male and female he created them.
(Genesis ch.1 vv. 26–7, ESV trans. Emphasis added)
Humanity’s appointment to rule over creation under God is the primary special characteristic of humanity. Our intellectual superiority is probably an important part of how we are to do this role well, but it is not essential. Under a Christian world view, if we were to find more intelligent life than ourselves, I think we would still have a responsibility to rule over those other creatures.
Human dominion is corrupted
As an aside, it is also important to recognise that power over others is frequently bad in our broken world. Because of our authority, our rebellion against God has broken the world. However, if we were exercising this authority correctly under God (rather than placing ourselves first), humanity’s dominion would be marked by care, love and wisdom—far different from the botched job we are doing. God knew our rulership would become toxic as we reject him, but it is still part of our purpose and we should aim to perform this better.
Separation by worship?
The whole creation seems to be involved in the worship of God. While one might interpret Jesus to be speaking in hyperbole, I think his statement in Luke about rocks crying out in praise might well be literal:
...the whole multitude of his disciples began to rejoice and praise God with a loud voice for all the mighty works that they had seen, saying, “Blessed is the King who comes in the name of the Lord! Peace in heaven and glory in the highest!” And some of the Pharisees in the crowd said to him, “Teacher, rebuke your disciples.” He answered, “I tell you, if these were silent, the very stones would cry out.”
(Luke ch. 19, vv. 37–40, ESV trans. Emphasis added)
Praise of God does not seem to be bound by intelligence or special appointment, but rather it is the domain of all created things, even the inanimate ones.
However, it also seems to me that there is something more immediate in our relation to God than the other members of creation have. Jesus became a human, not just any created thing. God also shows a plan to dwell with his people. We do seem to retain a privileged description in the New Creation as a kingdom of priests. Yet the great function of humanity, to love God and enjoy him forever, seems to be something true for more than just ourselves.
Conclusion
God has chosen to make humanity important, chosen that we should have authority over creation, and chosen that we should dwell with him. Everything that is special and distinct about humanity comes from God. We are not divine beings except where and to the extent that God makes us so.
I do not think that other intelligent life would be a defeater for Christianity. Other intelligent life, as scary as it might be, does not necessarily displace humanity’s position in Christianity. We have been appointed to our exalted station and it is not necessarily a meritocratic appointment. Our value as humans is not derived from abilities, but from God’s choice to make us so important.