That God, accepting He exists based on the previous proofs, is not a composite object won’t be especially difficult to believe. Except for those who, strangely, believe God is a created (or maybe “evolved”) being. If He was, it begs the question how. And that would immediately bring us back to how anything changes, which must involve the existence of a necessary Being, which is to say, God Himself. Reminder: it simply makes no sense to say things “just happen” or happen “by chance” or “randomly.” There must be a First Cause.
Chapter 18: That in God there is no composition
1 FROM the foregoing we are able to conclude that there is no composition in God. For in every composite thing there must needs be act and potentiality: since several things cannot become one simply, unless there be something actual there and something else potential. Because those things that are actually, are not united except as an assemblage or group, which are not one simply.i In these moreover the very parts that are gathered together are as a potentiality in relation to the union: for they are actually united after being potentially unitable. But in God there is no potentiality.ii Therefore in Him there is no composition.
2 Again. Every composite is subsequent to its components. Therefore the first being, namely God, has no component parts.
3 Further. Every composite is potentially dissoluble, so far as its composite nature is concerned, although in some there is something else incompatible with dissolution. Now that which is dissoluble is in potentiality to not-being. But this cannot be said of God, since of His very essence He is necessarily. Therefore there is no composition in Him.iii
4 Moreover. Every composition requires a compounder: for if there be composition, it results from several things: and things that are several in themselves would not combine together unless they were united by a compounder. If then God were composite, He would have a compounder: for He could not compound Himself, since no thing is its own cause, for it would precede itself, which is impossible. Now the compounder is the efficient cause of the composite. Therefore God would have an efficient cause: and thus He would not be the first cause, which was proved above.iv
5 Again. In any genus the more simple a thing is the more excellent it is; such, in the genus hot, is fire which has no admixture of cold. Therefore that which obtains the summit of nobility among beings, must be in the summit of simplicity. Now that which obtains the summit of nobility in things is what we call God, since He is the first cause, because the cause is more excellent than its effect. Therefore there can be no composition in Him.v
6 Moreover. In every composite thing the good does not belong to this or that part but to the whole, and I speak of good in reference to that goodness which is proper to, and is the perfection of, the whole: thus the parts are imperfect in relation to the whole: thus the parts of a man are not a man, nor have the parts of the number six the perfection of six, nor do the parts of a line attain to the perfection of the measure found in the whole line.vi Therefore if God is composite, His proper perfection and goodness are found in the whole of God but not in any of His parts.vii And thus the good that is proper to Him will not be purely in Him; and consequently He will not be the first and supreme good.viii
7 Further. Before every multitude it is necessary to find unity. Now in every composite there is multitude. Therefore that which is before all things, namely God, must needs be devoid of all composition.ix
iIf A is joined to B, there is A+B not AB, or rather, there is not an indivisible (new) C. If something is composite, it is in potential to being busted apart, to being A and B again.
iiDon’t forget that St Thomas earlier proved that in God there is no potentiality. Maybe it’s not obvious, but from this we deduce that God is not a “life-force”, or evolved being, made of parts. God is not an anthropomorphic being, is not made of pasta or DNA or anything material, even though he can be painted that way. God is Being itself, and Being itself is not be decomposable. Being itself cannot be painted or pictured.
iiiWe have already proved that God, as First Mover or Unchanging Changer, must necessarily exist, or nothing could ever move or change. This why St Thomas says, “since of His very essence He is necessarily.”
ivI find this very pretty. Everything that changes has a cause for the change. A composite is caused to be composite from its parts (somehow). And we’re right back at the beginning. Of course, we must never forget, not everything changes. God does not, because God is not in potentiality.
vThis extra, unneeded argument will probably sound phony, or at least fishy, to modern ears. And anyway, it doesn’t reach syllogistic proof because of the (as yet unproved) premise that the more excellent something is, the simpler it is. Simpler? To moderns, simpler is equated with stupider or that which is less useful. But to St Thomas, it is associated with elegance, beauty, sublimity. This is why he says “the cause is more excellent than its effect.” After all, without the cause, there is no effect.
viAnd here is similar language. A jigsaw puzzle can still be beautiful even though it’s missing a piece, but it hasn’t reached it’s potential perfection, or rather completeness. It may also come as a surprise to some atheists to learn that the parts of a man are not a man. But a man can have missing parts and still be a man, though an imperfect instantiation of one. A few cells which live as a man, though small, is also a man (after all, what are you but a large collection of cells?). That some objects, or people, exist as imperfections does not do away with perfection.
viiIn other words, if God is put together from pieces, those individual pieces are not the perfection—only the whole is. or could be Just like the jigsaw puzzle.
viiiThe good will not be purely in him, because the good would be in the whole which is made of pieces. The good would be in pieces too, as it were. Now this follows: “consequently He will not be the first and supreme good”. This is not a complete argument that God is the first and supreme good. St Thomas is anticipating that claim, which is anyway familiar even to us moderns (that God is Good). Thus, this also is not, at this point, another proof that God is not composite.
ixOkay, fine, But what about the Trinity? God the Father, Jesus-slash-God, God the Holy Ghost? How could God be three in one? How could a non-composite God be a man? Short answer: nobody knows. Not how. But we can know that. And we can only know things like this through faith, through revelation. Because of this, St Thomas does not concentrate on these matters, taking as his subject on those propositions which can be proved via the senses in absence of ordinary revelation.
We still haven’t close to describing God’s essence. We know he’s outside of time (eternal), not in potentiality, and not made of stuff. But that’s it so far. There is much more to come. How much? An infinite amount!
Update Perhaps this is the best ever explanation of the Trinity. Ch. xvi.
 Ch. xiii.
 Ch. xiii.