“Without division or diminution,” i.e. the generation of the Son entails no division or partition of the Godhead, still less any diminution of it. The Father is none the less God. His Godhead loses nothing by His begetting His Eternal Son. Some manuscripts have “demutatam” instead of “deminutam” here—i.e. “changed” for “diminished.” Certainly the begetting of the Son can make no change whatever in the Being of the Father, for the Divine Generation is “from everlasting to everlasting,” and is necessarily implied in the very Fatherhood, the personal essence of the Father. Hurter quotes St. Hilary, De Trin. 6, 10. “The Church knows of no apportionment made to the Son, but knows Him as perfect God of perfect God, as One begotten of One, not shorn off from Him, but born: she knows the Nativity to entail no diminution of Him Who begets, nor weakness in Him Who is born.” The fact is a spiritual relation, metaphysical in the highest sense, transcending our intelligence, and only to be apprehended by faith, simply as a fact—as the ἀρχή, or principle, which is sufficient for us. The “how” we must wait to have revealed to us hereafter, if we shall ever be able to receive it.
Editorial footnote · Exposition of the Christian Faith, Chapter XV., editorial note cite_note-2 · Original source