Though Aristotle clearly wants to prove the existence of God in the physics, it’s not clear exactly where he sees the proof as done. For St. Thomas, it seems to suffice to prove that there is some first mover of all things in motion. St. Thomas is probably thinking something like this: Whatever is natural is in motion, but the first mover is not in motion, so the first mover is something beyond nature. Joseph Bobik objects to this by pointing out that both Aristotle and St. Thomas thought that there were supernatural agents other than God, but St. Thomas seems to be able to counter this pretty easily – his whole account of motion is one that appeals to the division of act and potential, and so even if we posit multiple supernatural movers the proof still concludes to the most actual of them.
But all this involves St. Thomas’s presentation of the proof as metaphysical, which is not the presentation Aristotle gives in Physics. In that text, Aristotle concludes to God’s existence precisely from concluding to a being with infinite power. Motion, so Aristotle argues, must be infinite and relative to one mover, and so there must be some one, infinite reservoir or arche of the actuality that must pre-exist all possibilities that could arise in an infinite time.
The reason why St. Thomas didn’t advance this proof in his own writings are well known – he thought we simply had no way to figure out whether the universe must be infinite in time or not, and so he thought that Aristotle’s proof fails at its first move. But St. Thomas’s reason for this, where it is not simply a critique of Aristotle’s arguments, seems to be theological, and so relies on a pre-established decision to see the world as a creation arising from an inscrutable decision that we can, at best, only be told about by God himself. As a matter of purely physical theory, therefore, the possibility of knowing whether the universe is finite or infinite cannot be ruled out, and many physical theories will appeal to the necessity of both hypotheses. Theologians ought to be ready to see God just as much in an infinite universe as a finite one. As far as I know, no one has yet tried to update Aristotle’s physical proof to apply to the various modern and contemporary hypotheses about the infinite universe.