How Are We Able To Speak About God? - Aquinas' Compendium of Theology Chs. 21-27

Chs 21-27 Compendium of Theology 
Here are seven short chapters from a lesser known work of Aquinas called the "Compendium of Theology." These chapters deal with how we predicate things of God. Predication is a term from logic which means to assert as true some quality or reality to the subject you are talking about. So, basically, how do we say things are God? Can we say things about God? In what way can we say things about God? 
To tie all of these chapters together into one explanation, Thomas consistently points out that God is simple, and this simplicity is going to prevent us from speaking about God in the normal way we speak about things here on earth. For something to be simple, it means that it does not have parts or distinctions within itself. It is hard to conceptualize this for us because everything in this universe is multiplied by potency and parts. God, though, is pure and infinite actuality which means that there are no parts or distinctions. 

But wait, how is it possible to speak of "different" qualities of God if there can be no distinctions between those qualities? And that's exactly the point that Thomas is trying to get across to the reader. We speak of different qualities of God because that's the perspective that we have of God. That's how things work in our universe full of matter and potency. In God all these qualities are unified into God's one being as something we cannot comprehend, as we have no experience of God's essence. We don't know what God is in himself. All we can do is try to make analogies from what's better known to us. Like seeing facets of a diamond and giving each of them a name while not realizing that the facets are simply part of one larger whole. 

Thus, the mode of predication that we have of God is that of analogy. We cannot say univocally or directly what God is because we have no experience of this. We also are not prevented from saying anything at all because, as Thomas points out, every perfection of an effect must preexist in its cause, and therefore being is not equivocal to God as though he were completely "other." Rather, there is a middle way in which we express something true about God, but express it in a lower mode that we are familiar with, but that doesn't actually express what God is in himself. 1
If we gather together the various points established thus far, we perceive that all perfections in God are in reality one. We have shown above that God is simple. But where there is simplicity, there can be no distinction among the perfections that are present. Hence, if the perfections of all things are in God, they cannot be distinct in Him. Accordingly they are all one in Him. 2
I hope that helps makes some sense of how we can and cannot talk about God! 
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2 - ch 22