Would finding aliens cause a problem for Christianity?

Not necessarily, and Christian thinkers have discussed this possibility seriously for a long time, not just since modern science fiction made it popular.

No core Christian doctrine specifically requires humanity to be the only life, or even the only intelligent life, in the universe. Some Christian thinkers, including the Vatican Observatory's own director in a widely reported 2008 interview, have said discovering extraterrestrial life wouldn't contradict Christian faith, since it wouldn't limit God's creative freedom.

Harder, genuinely unresolved theological questions would follow: if intelligent life exists elsewhere, would it also need redemption? Would it be "fallen" the way humanity is? Would Christ's incarnation and atonement relate to it somehow, or would God's relationship to other beings work entirely differently? Writers like C.S. Lewis explored these questions seriously in both fiction and essays, without claiming certainty. This remains genuinely open, speculative theology—interesting to think through, not something requiring settled answers in advance.