See the first post in this series for an explanation and guide of our tour of Summa Contra Gentiles. All posts are under the category SAMT.
This may be proved in three ways. The first…
Perhaps a simple way of summarizing this chapter is that (what is not surprising) God is unlike anything else, and our language, properly used, necessarily must reflect this. I’m trying out the new footnote style here, too.
Chapter 32: That Nothing Is Predicated Univocally Of God And Other Things
1 FROM the above it is clear that nothing can be predicated univocallyi of God and other things. For an effect which does not receive the same form specifically as that whereby the agent acts, cannot receive in a univocal sense the name derived from that form: for the sun and the heat generated from the sun are not called hot univocally. Now the forms of things whereof God is cause do not attain to the species of the divine virtue, since they receive severally and particularly that which is in God simply and universally. It is evident therefore that nothing can be said univocally of God and other things.ii
iUnivocally: having one only one unambiguous meaning. Aristotle: “A man and an ox are both animal, and these are univocally so named, inasmuch as not only the name, but also the definition, is the same in both cases: for if a man should state in what sense each is an animal, the statement in the one case would be identical with that in the other.”
This is contrasted with equivocally; e.g. saying “he’s tall”, “that’s a tall order” use tall equivocally. Don’t argue with me, this article is an argument!
iiIt’s obvious the sun and the heat we feel from it, though both are hot, are not the same thing. We must use hot equivocally. As we’ll see below, the things that we can say about God we can’t say about things which aren’t God.
Here is St Thomas on the same subject in Summa Theologica (emphasis mine):
…In the same way, as said in the preceding article, all perfections existing in creatures divided and multiplied, pre-exist in God unitedly. Thus when any term expressing perfection is applied to a creature, it signifies that perfection distinct in idea from other perfections; as, for instance, by the term “wise” applied to man, we signify some perfection distinct from a man’s essence, and distinct from his power and existence, and from all similar things; whereas when we apply to it God, we do not mean to signify anything distinct from His essence, or power, or existence. Thus also this term “wise” applied to man in some degree circumscribes and comprehends the thing signified; whereas this is not the case when it is applied to God; but it leaves the thing signified as incomprehended, and as exceeding the signification of the name. Hence it is evident that this term “wise” is not applied in the same way to God and to man. The same rule applies to other terms. Hence no name is predicated univocally of God and of creatures.
…4 Again. That which is predicated univocally of several things is more simple than either of them, at least in our way of understanding. Now nothing can be more simple than God, either in reality or in our way of understanding. Therefore nothing is predicated univocally of God and other things.iii
iiiDon’t forget simple is a technical term here, meaning not made of parts, with no potential, etc. Whatever we can discover to say univocally about God can’t be said of anything else in that same way.
5 Further. Whatever is predicated univocally of several things belongs by participation to each of the things of which it is predicated: for the species is said to participate the genus, and the individual the species. But nothing is said of God by participation, since whatever is participated is confined to the mode of a participated thing, and thus is possessed partially and not according to every mode of perfection. It follows therefore that nothing is predicated univocally of God and other things.iv
ivA gross simplification, or perhaps a good parlor game: see if you can name of thing predicated of God which cannot be predicated of anything else. For example, “God is being-itself” which predicates the being-itselfness of God. But other things certainly have being (you do, I do), though nothing else is being-itself. Inasmuch as we can comprehend this term (which is a long way short of the fullness of it), we can only say this univocally of God and of nothing else.
6 [THIS ARGUMENT MAY BE SKIPPED] Again. That which is predicated of several things according to priority and posteriority is certainly not predicated of them univocally, since that which comes first is included in the definition of what follows, for instance substance in the definition of accident considered as a being. If therefore we were to say being univocally of substance and accident, it would follow that substance also should enter into the definition of being as predicated of substance: which is clearly impossible.v Now nothing is predicated in the same order of God and other things, but according to priority and posteriority: since all predicates of God are essential, for He is called being because He is very essence, and good because He is goodness itself: whereas predicates are applied to others by participation; thus Socrates is said to be a man, not as though he were humanity itself, but as a subject of humanity. Therefore it is impossible for any thing to be predicated univocally of God and other things.vi
vIt would be circular.
viA last emphasis. If we can speak univocally of God, whatever term we happen to use it would then be impossible to use it univocally of any other thing.
 Chs. xxviii., xxix.
 Ch. xxiii.
 Chs. xxiv., xxv.
 Ch. xxiii.