9 For instance, these heretics would have it that the Son of God is not from God; that God was not born from God out of, and in, the nature of God. To this end, when they have solemnly borne witness to “One God, alone true,” they refrain from adding “The Father.” And then, in order to escape from confessing one true Godhead of Father and of Son by a denial of the true birth, they proceed, “Not, as Valentinus maintained, that the Son is a development of the Father.” Thus they think to cast discredit upon the birth of God from God by calling it a “development,” as though it were a form of the Valentinian heresy.
For Valentinus was the author of foul and foolish imaginations; beside the chief God, he invented a whole household of deities and countless powers called æons, and taught that our Lord Jesus Christ was a development mysteriously brought about by a secret action of will. The faith of the Church, the faith of the Evangelists and Apostles, knows nothing of this imaginary development, sprung from the brain of a reckless and senseless dreamer. It knows nothing of the “Depth” and “Silence” and the thrice ten æons of Valentinus.
It knows none but One God the Father, from Whom are all things, and One Jesus Christ, our Lord, through Whom are all things, Who is God born from God. But it occurred to them that He, in being born as God from God, neither withdrew anything from the Divinity of His Author nor was Himself born other than God; that He became God not by a new beginning of Deity but by birth from the existing God; and that every birth appears, as far as human faculties can judge, to be a development, so that even that birth might be regarded as a development.
And these considerations have induced them to make an attack upon the Valentinian heresy of development as a means of destroying faith in the true birth of the Son. For the experience of common life leads worldly wisdom to suppose that there is no great difference between a birth and a development. The mind of man, dull and slow to grasp the things of God, needs to be constantly reminded of the principle, which I have stated more than once, that analogies drawn from human experience are not of perfect application to the mysteries of Divine power; that their only value is that this comparison with material objects imparts to the spirit such a notion of heavenly things that we may rise, as by a ladder of nature, to an apprehension of the majesty of God.
But the birth of God must not be judged by such development as takes place in human births. When One is born from One, God born from God, the circumstances of human birth enable us to apprehend the fact; but a birth which presupposes intercourse and conception and time and travail can give us no clue to the Divine method. When we are told that God was born from God, we must accept it as true that He was born, and be content with that. We shall, however, in the proper place discourse of the truth of the Divine birth, as the Gospels and the Apostles set it forth. Our present duty has been to expose this device of heretical ingenuity, this attack upon the true birth of Christ, concealed under the form of an attack upon a so-called development.
Source: On the Trinity (New Advent)