Real Relations exist in the Divinity. For wherever anything proceeds from a principle of like nature, it is necessary that the thing proceeding and the source from which it proceeds should belong to the same order, and so bear real relationship the one to the other; and since, in the Divinity, Procession is in identity of Nature, Real Relations are to be accepted.
This Relationship which exists in God is, in reality, one and the same with His Essence, and differs only according to the conception of our intelligence; for whatever has accidental being in created things, when transferred to God, in our mind has substantial being. Nevertheless these Relations, in God, are really distinguished from each other; for the multiplicity of the Trinity is due to Relation only: therefore, if these Relations were not really distinct from each other, the Trinity would not be real, but only conceptional, which was the error of Sabellius. Because, however, there is in God the highest Unity and real Simplicity, this distinction does not exist according to the Divine Essence considered absolutely, but only relatively. Thus Boethius says that in the Divinity, Substance contains Unity, while Relation multiplies in Trinity.
Nor are there other Relations in the Divine Essence except those of Paternity, Filiation, Spiration and Procession; for Relations, in God, are not consequent upon quantity or external action; hence the relations of God to creatures are not real in Him. The Divine Relations are to be understood, therefore, according to actions within the Godhead, which are two: one according to the intellect, namely, the Procession of the Word, which is Generation, to which corresponds Paternity; the other according to the Will, which is the Procession of Love. This has no name proper to it; but the Relation of the Principle we call Spiration, and that of the Person Proceeding, Procession.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni