Before the jews got serious, absolutely no-one thought that there was only one god. All previous ideas about gods were (as far as we know) of the Conan the Barbarian variety*, limited in scope and basically unimaginative, admitting of the possibility of others in the way that old maps admitted of the possibility that there were continents that weren't shown on the map.
The idea that the jews had about the all-encompassing nature of their god was a new one, and one of the more significant ideas that humans have had (in terms of its impact rather than its correctness necessarily). There is no room at all in the theology of the abrahamic religions for any other gods to be out there. There is no space in which they can exist.
[in this view] If there are aliens out there they are a part of god's creation just as much as we are. Whether they have the same insights into god as we do doesn't seem to matter. But I suppose if we could have any meaningful relationship with them we might easily say that they too were created in god's image, we'd just have to assume (as I think most grown-ups do) that this has to do with characterisitcs other than having tentacles.
I'm reading Paul Davies book "the eerie silence" at the moment. I love the fact that there are serious, intelligent people out there actually making plans for dealing with alien contact if it happens. 🙂
* Conan: What gods do you pray to?
Subotai: I pray to the four winds… and you?
Conan: To Crom… but I seldom pray to him, he doesn't listen.
Subotai: [chuckles] What good is he then? Ah, it's just as I've always said.
Conan: He is strong! If I die, I have to go before him, and he will ask me, "What is the riddle of steel?" If I don't know it, he will cast me out of Valhalla and laugh at me. That's Crom, strong on his mountain!
Subotai: Ah, my god is greater.
Conan: [chuckles] Crom laughs at your four winds. He laughs from his mountain.
Subotai: My god is stronger. He is the everlasting sky! Your god lives underneath him.