In natural theology, it is almost a given that more can be said by way of what God is not than what He is. This is the via negativa, the negative way. Of course, in Scripture, we can know things about God that we cannot know through reason alone, and among these is, of course, the Trinitarian nature of the one God. Still, Scripture often emphasizes those things that God is not, helping us to avoid comparing the Creator too directly with creation, where the temptation to pantheism, idolatry, and other dangers would creep in. For “whatever is said of a nature, unchangeable, invisible and having life absolutely and sufficient to itself, must not be measured after the custom of things visible, and changeable, and mortal, or not self-sufficient.”
Augustine certainly emphasizes this via negativa, even while restating his belief that the Trinity is something that God has positively revealed to us. “Whoso thus thinks of God, although he cannot yet find out in all ways what He is, yet piously takes heed, as much as he is able, to think nothing of Him that He is not.”
Augustine goes on to speak of the ways in which we may speak of God. He is unchangeable, and thus, anything we say “happens to Him” or any human emotion we apply to Him is at best analogous, but more often than not, is actually expressing a change in creation. For example, when God becomes angry with man, it is really that man has positioned himself differently toward the divine justice, for it is not God’s “emotions” that changed but man is the “relative partner.”
All that is “in” God is substantial, and never accidental (metaphysically speaking), and all things describing God must be viewed either as an aspect of this divine simplicity or otherwise as a metaphor for our understanding. Either way, nothing can contradict this supreme simplicity and oneness of God and be taken in a literal way.
However, within the Godhead, it is not always true that everything is said substance-wise. The one category, the one “accident” that is applicable to God is the unique category of relation, and it is by this that we begin to have an understanding of the Son as related to the Father, and likewise with the three Persons of the Trinity. Of course, the Son differs in no way substantially (as substance) from the Father or the Spirit, but only by relation.
The remainder of Book V continues to look as substance, relation, person, and other words and the way they must be used and must not be understood when speaking of God, both as related to us but most especially as these words describe, as best as human language can, the immanent life of God.
After all, our entire language (any human language) is one built around the idea of expressing changeable being within time. The jump from this language to things such as metaphysics takes great precision in speech and depth of thought. Even further, then, is the stretch of human language to the Creator Himself, who is beyond even the subject of metaphysics but is, rather, its principle.