Christians might have reasons to suspect that aliens don’t exist – one could reasonably expect God to say such a thing if it were true in His holy Word – but naturalists generally have no need to reject the possibility of E.T. existence. There might be naturalists who personally find it unlikely, but very few would claim that it is impossible or that belief in aliens is like believing in Santa.
However, this seems to be incompatible with naturalism. If there is no way for an atheist to exclude the possibility of intelligent life in our universe, how can s/he exclude the existence of intelligent, spiritual life outside of it? Openness to alien life seems to be necessarily combined with an openness to the supernatural.
The naturalist might respond that we have no objective reference point to non-physical persons, while aliens can be viewed as simply animals that happened to evolve elsewhere. To this objection I would point out that we can imagine vastly different aliens in out universe, and if we add possible parallell universes – that most naturalists are open to – there’s really no limit to what creatures we can conceive as possible.
Hence, openness to alien life seems not to rest on references to earthly animals but on what’s logically possible. And there is no logical contradiction in imagining non-physical, spiritual beings. To those who think that non-physical beings are impossible I would simply point out that that’s a circular argument that presupposes naturalism.
We have no evidence for the existence of alien life, or the existence of other universes. Yet most atheists would say that it’s entirely possible for aliens or other universes to exist. I find it arbitrary to view the existence of supernatural entities as impossible due to (perceived) lack of evidence when one does not draw that conclusion concerning aliens.
So just to be clear: I’m not saying that I know that aliens exist, or that God is an alien. What I’m saying is that a naturalist either must say that alien life is impossible, or cease to be a naturalist. There is no middle ground.