The thing is Aquinas says that an infinite regress IS possible, but it is shown by divine revelation that God created us and there is a beginning. I don’t remember where this is found but it is out there.
When Aquinas in this talks about this can’t go on to infinity he means that an infinite regress can’t exist in itself. Something that is always active must be responsible for things that are in motion.
Atheist always tend to use the who created God argument. Many Christians can’t respond to it, because they have a Kantian view of the world. A causes B is the only cause that exists. Many don’t have an understanding of primary causality, formal causality final causality etc.
To be fair, I think Aquinas would allow that an infinite regress could be possible, but that it couldn’t sufficiently explain its own existence or even how an Uncaused Cause (aka God) could affect the present from a time in the infinite past. Hilbert’s writings confirm this.
In other words, for anyone looking, a la the principle of sufficient reason, for a sufficient explanation for a causal chain within the chain itself, an infinite regression would be ruled out in principle. You could not, by definition have a sufficient reason for an infinite chain of events within that very chain of events.
That does not rule out the possibility of infinite causal chains existing if the sufficient reason for the existence of the chain itself is “outside” the chain.
This is where Aquinas gets into the difference between causes per se and per accidens
Take the train example.
It is true that the motion of the cars currently in front of us (the present) would never have been passed to these cars if the initial movement began an infinitely long time ago since, for one, infinity would rule out “initial” by definition.
What Aquinas is saying is that a chain of accidental “motion” or change could not depend upon an initiator of that motion from the infinite past.
However, that does not, by itself, rule out the possibility of an infinite chain of “motion” if the initiator is not in the chain itself, but outside of it.
Take the infinitely long train. It could not be powered by a locomotive at one end, partly because there is no end to begin with, but also because “motion” could not be transmitted from the infinite past to the present - it would never get here.
However, if what it is that “powers” the chain of train cars is not a locomotive at the end but, if this were, say, a child’s play set consisting of an infinitely long train on an infinitely long track, the child’s hand at a specific location in time and space could by pushing it from somewhere in the middle, transfer motion in both directions (front and back) at the same time. Similarly some conception of the “hand of God” (Unmoved Mover, motion per se, etc.) could effect the motion of the train from the present in both directions - from the past and into the future - moving the train forward. In other words, if the “motion” or “cause, per se, of motion” were imparted in the present there would be no need to overcome the problem of an actual infinite and Aquinas would be quite correct to admit the conceptual possibility of an infinite universe provided the present was key to explaining the “motion” of the universe in both temporal directions, i.e., the infinitely long train is moved “from the middle,” from the present by a cause of motion “outside” of the train itself.
The problem with this analogy is that the child’s hand only explains one aspect of the train (its motion in the commonly held sense,) whereas the principle of sufficient reason requires that everything about the train (size, shape, colour, location, existence, etc.) be explained, but a “past dependent” explanation can never suffice for the same reason a past dependent explanation for motion doesn’t.
When Aquinas insists, for example, that God “sustains” all of creation in existence, he is maintaining that the subsistent cause of reality is from the eternal “Now,” not from “the past” pushing forward. This means that causal chains could conceivably be of infinite length in either direction since time, itself, could be generated atemporally.
That doesn’t mean that infinite causal chains necessarily exist, but merely that they are not ruled out a priori. Current Big Bang cosmology is demonstrating that the universe itself is not an infinite causal chain.
Either way, whether the universe is infinite or not, Aquinas’ argument is not affected because he demonstrates that any cause, to sufficiently explain what is required of it, cannot merely operate within either an infinite or finite chain since an Uncaused Cause (Unmoved Mover) must logically account for its own existence (why anything exists at all) prior to (and independent of) explaining anything else.
I think you misunderstand Kreeft, by the way.
, seem to miss. It could be that they are talking to a modern argument so they are mixing it in with their arguments.