It's better that we start with a high-level overview of the existence of God, rather than getting too detailed in the specific arguments. Then readers will have a general idea of what is supposed to be going on with this evidence.
We saw that most things are composites. Not just physical composites (an elephant is composed of organs, which are composed of cells, etc), but more fundamental composites as well, such as the actual and possible distinction. The coffee cup in front of you is a composite of the way it actually is right now (filled with coffee and on your desk) and the way it possibly could be (empty and in the sink).
If something is a composite, then it can't bring its own parts together in order to exist, because that would mean that it simultaneously exists (so that it can pull its own parts together) and not exist (because it doesn't exist without being put together).
So if something is composite, it must be held together by something else.
But that something else is either A) composite, or B) non-composite.
If B, then it needs something to hold it together as well and the chain continues. Now you see we are in the same situation as we described in the last post.
Which means that there must be something that is not composite that "holds the whole stack together", so to speak. Something that doesn't need to be held together by anything else because it isn't composite. For example, if X is not composite, then it can hold together D, which in turn holds together C, which holds together B, which holds together A. If X were removed, the whole stack would fall apart.
So if that something isn't composite, then it isn't physical, because physical things are made of either parts, or they are located somewhere in space. Which makes them movable (at least in principle) and thus a mixture of actual and possible.
It isn't energy either, since energy can change location, and is thus a mixture of actual and possible.
In fact, the non-composite thing could only be either A) just actual, or B) just possible. But something cannot be just possible, with no actual existence. That would mean that it both doesn't exist (because it has no actuality), and does exist (because it is an existing thing that is just possible), which is a logical contradiction.
So the non-composite thing would have to be just actual.
Why should we label something that is purely actual "God"? Find out in the next post.