Aition: a tale devised to explain the origin of a religious observance
Even from the perspective of fundamentalist Christianity, which I'd hazard a guess many people would tend to assume to be the most hostile to the idea, there's no reason to believe there aren't aliens somewhere out there. It gets more complicated if fallen humanity is allowed to interact with those other species, but even then you can argue either way within that axiom set.
Really, I'd say the reason that most of us, religious or otherwise, believe "we are the only life in the universe" (or "observable universe") is because that's the simplest explanation that fits the facts. It may well be wrong, but at the moment, there's not even a whiff of convincing evidence that we are not alone, even as we must also all acknowledge there's plenty of room in the "gaps" for alien life, even intelligent alien life, even intelligent alien life that spans several star systems. It depends on how far away we're talking, but even fairly close to home it could take a Kardashev level 2.3-ish at a minimum for us to be sure. No matter what your ideological disposition may be, there's just a huge "NO DATA" out there right now.
That's a religious argument. It's unprovable and unknowable.