As the early church began to formulate its language about the Trinity, they needed to explain how we can say that there is one God in three Persons and how those Persons relate within the Godhead. In order to fight against heresies like Arianism, the orthodox Christians established that there are eternal relations of origin: the Father is unbegotten, the Son is begotten of the Father, and the Holy Spirit proceeds from the Father and Son. In other words, there is a fitting way to explain the order (“taxis”) within the Godhead that does not imply that any of the Persons are inferior to one another or different from one another in divine essence. In On Not Three Gods, Gregory of Nyssa explains how this doctrine works:
If, however, any one cavils at our argument, on the ground that by not admitting the difference of nature it leads to a mixture and confusion of the Persons, we shall make to such a charge this answer;—that while we confess the invariable character of the nature, we do not deny the difference in respect of cause, and that which is caused, by which alone we apprehend that one Person is distinguished from another;—by our belief, that is, that one is the Cause, and another is of the Cause; and again in that which is of the Cause we recognize another distinction. For one is directly from the first Cause, and another by that which is directly from the first Cause; so that the attribute of being Only-begotten abides without doubt in the Son, and the interposition of the Son, while it guards His attribute of being Only-begotten, does not shut out the Spirit from His relation by way of nature to the Father.
But in speaking of “cause,” and “of the cause,” we do not by these words denote nature (for no one would give the same definition of “cause” and of “nature”), but we indicate the difference in manner of existence. For when we say that one is “caused,” and that the other is “without cause,” we do not divide the nature by the word “cause1322”, but only indicate the fact that the Son does not exist without generation, nor the Father by generation: but we must needs in the first place believe that something exists, and then scrutinize the manner of existence of the object of our belief: thus the question of existence is one, and that of the mode of existence is another. To say that anything exists without generation sets forth the mode of its existence, but what exists is not indicated by this phrase. If one were to ask a husbandman about a tree, whether it were planted or had grown of itself, and he were to answer either that the tree had not been planted or that it was the result of planting, would he by that answer declare the nature of the tree? Surely not; but while saying how it exists he would leave the question of its nature obscure and unexplained. So, in the other case, when we learn that He is unbegotten, we are taught in what mode He exists, and how it is fit that we should conceive Him as existing, but what He is we do not hear in that phrase. When, therefore, we acknowledge such a distinction in the case of the Holy Trinity, as to believe that one Person is the Cause, and another is of the Cause, we can no longer be accused of confounding the definition of the Persons by the community of nature.
Thus, since on the one hand the idea of cause differentiates the Persons of the Holy Trinity, declaring that one exists without a Cause, and another is of the Cause; and since on the one hand the Divine nature is apprehended by every conception as unchangeable and undivided, for these reasons we properly declare the Godhead to be one, and God to be one, and employ in the singular all other names which express Divine attributes.
This will appear in the forthcoming CSB Ancient Faith Study Bible.