35 For God is not born from God by the ordinary process of a human childbirth; this is no case of one being issuing from another by the exertion of natural forces. That birth is pure and perfect and stainless; indeed, we must call it rather a proceeding forth than a birth. For it is One from One; no partition, or withdrawing, or lessening, or efflux, or extension, or suffering of change, but the birth of living nature from living nature. It is God going forth from God, not a creature picked out to bear the name of God. His existence did not take its beginning out of nothing, but went forth from the Eternal; and this going forth is rightly entitled a birth, though it would be false to call it a beginning. For the proceeding forth of God from God is a thing entirely different from the coming into existence of a new substance. And though our apprehension of this truth, which is ineffable, cannot be defined in words, yet the teaching of the Son, as He reveals to us that He went forth from God, imparts to it the certainty of an assured faith.
Source: On the Trinity (New Advent)