Augustine’s rules for theological discourse in Book 5 of De Trinitate give us a grammar to speak of the mystery of God. They protect us from heresy, draw our minds to the meaning of Scripture, and allow us to know God with clear concepts.

Augustine’s 3 Rules for Fitting Language of God

In the fifth book of his De Trinitate, Augustine shifts his theological focus to what transcends human comprehension. As he says: “From now on I begin to speak of subjects, which are altogether above the power of any man” (Aug., De Trin. 5.1.1).

What follows in Book 5 then is Augustine’s attempt to develop a theological grammar adequate for discussing both the divine nature and the relation of the Father, Son, and Spirit. He discovers three rules that should rule our speech about God.

Rule 1: We can’t judge uncreated nature by created things

Augustine’s first rule challenges the direct application of earthly categories to divine realities. He asserts: “whatever is said about a nature invisible, unchangeable, with the highest kind of life and sufficient to itself, ought not to be judged by the ordinary standard of visible, changeable, mortal, or dependent things.” (Aug., De Trin. 5.1.2)

This rule guards against anthropomorphic or materialistic conceptions of divinity. In other words, created things cannot be univocally applied to the uncreated Creator.

Rule 2: We mustn’t confuse substantial and relational words

Augustine’s theological grammar secondly distinguishes between substantial and relational predication:

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