If aliens exist as we think of them in science-fiction, sensient beings, they will be humans in the broad sense of the word, even if they have tentacles instead of legs and arms, and therefore, fallen.
But I'm not optimistic about this. If they exist, it means that either no species has achieved interstellar/intergallatic travel capability, or that it is, in fact, impossible for biological organisms.
My leanings, though, are with the Rare Earth hypothesis. ( http://en.wikipedia.org/wiki/Rare_Earth_hypothesis
I might concede alien races *with* the Rare Earth hypothesis if we take into account the possibility of a Multiverse. Aliens would exist in other universes and that would be the source of the difficulty in communication.
*If* we are the only intelligent race in this universe - as I guess we are - then, the *whole* universe is ours to explore. If that is the case, "taking the Gospel to all nations" actually mean spreading it, literally, through all the inhabitable worlds in the universe and even those that can be made inhabitable through terraformation. If the Day of Judgment will come only after that, than current calculations of the scientists for the death of the universe may allow for a "human diaspora" throughout the Universe before that.