Terms to be defined:
Act: That which, in the time and respect being discussed, is the case.
Potency: That which, in the time and respect being discussed, may be the case but is not.
Whatever changes has its origins in that which exists, because only that which exists can be causally changed or effect a causal change. Therefore, every change is an incident of act realizing potency.
Now, a thing cannot be in a state of act and potency at the same time and in the same respect. Therefore, anything which changes has its origin in some antecedent cause of act.
For anything which has extension in spacetime, there are two possibilities regarding any fact about it: It was caused to be this way, or it always was this way.
If it always was this way, there must have been an infinite regress in time. However, an actual infinite cannot be completed by successive addition. The temporal events of the past have been completed by successive addition. Therefore, the temporal events of the past are not an actual infinite. As such, anything which has extension in spacetime must admit of causal explanation.
Now as we have seen, the temporal events of the past cannot form an actual infinite. Therefore the causal chain of the past cannot form an actual infinite. Therefore, there must be some first cause.
Now since every causal relationship is an example of act realizing potency, a first cause, in order to be able to cause without having been caused, must be an entity of pure act. An actus purus. What are the attributes of an actus purus?
As we have seen, an actus purus cannot have extension in spacetime, because whatever has extension in spacetime admits of causal explanation. But that which contains no potency admits of no causal explanation.
There could only be one such entity. When we say that "An apple exists" and that "A horse exists" we mean the same thing by the word "exists". As such, existence does not differ from existence qua existence. So an actus purus admits of no differentiation, and there can only be one.
An actus purus, containing no potency, must be both perfect and perfectly simple, as it could not exist in any way other than just how it does exist.
Finally, an actus purus is not subject to the laws of cause and effect, as these laws govern the realization of potency by act, but an actus purus has no potency.
Thus we can know of the existence of a single entity, lacking extension in spacetime, which is perfect, perfectly simple, and immune from the laws of cause and effect, and which forms the first cause from which all other causes flow. This is the entity men call God.