Similarly the Father is truth, the Son is truth, and the Holy Spirit is truth; Father, Son, and Holy Spirit are not three truths, but one truth. If, then, they are one substance and one truth, truth must of necessity be a substantial predicate. So Goodness, Immutability, Justice, Omnipotence and all the other predicates which we apply to the Persons singly and collectively are plainly substantial predicates. Hence it appears that what may be predicated of each single One but not of all Three is not a substantial predicate, but of another kind--of what kind I will examine presently. For He who is Father does not transmit this name to the Son nor to the Holy Spirit. Hence it follows that this name is not attached to Him as something substantial; for if it were a substantial predicate, as God, truth, justice, or substance itself, it would be affirmed of the other Persons.
Similarly the Son alone receives this name; nor does He associate it with the other Persons, as in the case of the titles God, truth, and the other predicates which I have already mentioned. The Spirit too is not the same as the Father and the Son. Hence we gather that Father, Son, and Holy Spirit are not predicated of the Divinity in a substantial manner, but otherwise.[27] For if each term were predicated substantially it would be affirmed of the three Persons both separately and collectively. It is evident that these terms are relative, for the Father is some one's Father, the Son is some one's Son, the Spirit is some one's Spirit. Hence not even Trinity may be substantially[28] predicated of God; for the Father is not Trinity--since He who is Father is not Son and Holy Spirit--nor yet, by parity of reasoning, is the Son Trinity nor the Holy Spirit Trinity, but the Trinity consists in diversity of Persons, the Unity in simplicity of substance.
Now if the Persons are separate, while the Substance is undivided, it must needs be that that term which is derived from Persons does not belong to Substance. But the Trinity is effected by diversity of Persons, wherefore Trinity does not belong to Substance. Hence neither Father, nor Son, nor Holy Spirit, nor Trinity can be substantially predicated of God, but only relatively, as we have said. But God, Truth, Justice, Goodness, Omnipotence, Substance, Immutability, Virtue, Wisdom and all other conceivable predicates of the kind are applicable substantially to divinity.