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cf., Sent.I.D9.Q1.A1, Sent.I.D19.Q1.A1.Rep2, Sent.I.D24.Q2.A1, QDePot.Q9.A8
In this article, St. Thomas begins by showing that we need to use proper terms befitting God. For, while terms may be within the scope of proper use, such terms, able to be piously explained, often can lead to heresy. For example, if I said "Jesus is not God," the judgement can be piously explained, i.e., "Jesus is not Divine in accordance with His humanity," yet, the judgement is more likely to be taken in the sense of a universal negation of Divinity in Jesus.
Further, in theology, we often are required to "go between" opposed errors. In De Deo Trino, these are the classically opposed errors of "Arianism" and "Sabellianism."
Thus, in this article, St. Thomas seeks how we are to avoid terms that are ordinarily taken in an Arian or a Sabellian sense, and establish affirmations and negations that are "between" (and above) these opposed errors.
As to Arianism, there are two negations and one affirmation. First, we say "there is no diversity in God." Second, we say "there is no difference in God." Third, we say "there is distinction in God," which followed upon relative opposition.
Yet, we must recognize that, often, the fathers did not speak with as much precision as later theologians. Thus, we do have instances of fathers using the terms "diversity" and "difference," yet, we ought to read them as meaning something different by that term, i.e., distinction.