"Must have a cause" is not a proof that it must. The first cause is a perfect example that we can believe something is without one, an exception.
The First Cause (which Aquinas says is God) eliminates this infinite regress because it is - by definition - uncaused.
How does he know it's God? If something we call God can be believed uncaused, then the universe can just as easily be assumed uncaused.
It requires no previous cause, no chain, and therefore does not infinitely regress
Except this is not through knowledge, but by our limited convention.
"Must have a cause" is not a proof that it must.
It's true the argument stands or falls on the truth of a cause and effect universe. However, if you are going to posit it's not true, you need to explain what we observe everywhere in the universe, as cause and effect, really is. Take any example - a billiard ball compressing a cushion - and explain what that event is if it is not cause and effect. Not saying that you can't, but that there's a hole in this objection without it.
The first cause is a perfect example that we can believe something is without one, an exception.
But we are not imagining the first cause as part of the universe (that we observe as cause and effect). The argument is that all things in motion (in the largest sense) must have cause and effect. The first cause is not in motion, is not changing, etc. - and therefore without need of cause.
We cannot imagine the same thing for the universe that we observe as in motion under cause and effect - without explaining away cause and effect as in your first observation.
How does he know it's God?
Personally, my view is that if you can fully comprehend it, it's not God; that first cause could only be a partial answer. However, answering in terms of the argument: If we are describing something that is eternal, changeless (i.e.,perfect) uncaused, immortal, self-existing, independent… we're pretty close to the describing the same thing Christians call God. You're correct though that the argument itself could be used without the term or hold my view that it's still incomplete, or that it does not describe/prove everything Christianity means by "God."
Thanks very much for your reply.