There is scarcely any doubt now remaining in the philosophical world that the moon is a habitable globe. The most accurate observations that have been made with the most powerful telescopes have confirmed the opinion. The moon seems, in almost every respect, to be a body similar to our earth; to have its surface diversified by hill and dale, mountains and valleys, rivers, lakes, and seas. And there is the fullest evidence that our earth serves as a moon to the moon herself, differing only in this, that as the earth's surface is thirteen times larger than the moon's, so the moon receives from the earth a light thirteen times greater in splendor than that which she imparts to us; and by a very correct analogy we are led to infer that all the planets and their satellites, or attendant moons, are inhabited, for matter seems only to exist for the sake of intelligent beings.There you have it: the classical Wesleyan theology of Star Trek! Now, if we can only get Cochran to build and fly his warp spaceship so the Vulcans can land for first contact and...
I'm not deriding the famed expositor, only poking gentle fun with him. Every preacher, teacher or scholar probably has a few things to be filed under "Speculative Theology". (Remember my post on Wesley and ghosts three days ago?)
We always have to be careful with our speculations, because speculations lead to theological ramifications that direct us toward unanswerable paradoxes or conundrums (or even heterodoxy).
I'll go out on a limb of my own: we human beings (as self-aware, intelligent creation) are alone in the universe. I believe it for theological reasons. Follow my train of thought.
1. At his incarnation, the Son of God also became Man. Jesus Christ, the God-Man, died on the cross for our sins.
2. At his resurrection, the Lord Jesus rose from the dead, the first-fruits of those who will rise.
3. Jesus remains the God-Man. Paul says explicitly, "For in him the whole fullness of deity dwells bodily" (Col 2.9 ESV)
This is crucial; the Greek for "dwells" (κατοικει) is present active indicative. In other words, he presently resides "bodily". Christ never shed his humanity and then, in some reverse-incarnation became "just" the Son of God again. He is still the God-Man, "bodily".
4. If Jesus is the God-Man then it is obvious that he couldn't incarnate into another species. Therefore, there could be no possible salvation for other groups on different planets in this universe (unless, of course, they remained sinless which, to me, seems dicey).
Am I speculating? Yes, I am! Is that dangerous? Yes, it can be! Does the Bible speak directly to this issue? If Paul doesn't speak directly to it in Colossians 2.9 then I doubt it's directly spoken to anywhere in Holy Writ.
But I still think my speculation is better informed than Adam Clarke's. ;-)