10 And then, in continuation of this same fraudulent assault upon the faith, their confession proceeds thus:— “Nor, as Manichæus has declared of the Son, a consubstantial part of the Father.” They have already denied that He is a development, in order to escape from the admission of His birth; now they introduce, labelled with the name of Manichæus, the doctrine that the Son is a portion of the one Divine substance, and deny it, in order to subvert the belief in God from God.
For Manichæus, the furious adversary of the Law and Prophets, the strenuous champion of the devil's cause and blind worshipper of the sun, taught that That which was in the Virgin's womb was a portion of the one Divine substance, and that by the Son we must understand a certain piece of God's substance which was cut off, and made its appearance in the flesh. And so they make the most of this heresy that in the birth of the Son there was a division of the one substance and use it as a means of evading the doctrine of the birth of the Only-begotten, and the very name of the unity of substance.
Because it is sheer blasphemy to speak of a birth resulting from division of the one substance they deny any birth; all forms of birth are joined in the condemnation which they pass upon the Manichæan notion of birth by severance. And again, they abolish the unity of substance, both name and thing, because the heretics hold that the unity is divisible; and deny that the Son is God from God, by refusing to believe that He is truly possessed of the Divine nature. Why does this mad heresy profess a fictitious reverence, a senseless anxiety?
The faith of the Church does, as these insane propounders of error remind us, condemn Manichæus, for she knows nothing of the Son as a portion. She knows Him as whole God from whole God, as One from One, not severed but born. She is assured that the birth of God involves neither impoverishment of the Begetter nor inferiority of the Begotten. If this be the Church's own imagining, reproach her with the follies of a wisdom falsely claimed; but if she have learned it from her Lord, confess that the Begotten knows the manner of His begetting.
She has learned from God the Only-begotten these truths, that Father and Son are One, and that in the Son the fullness of the Godhead dwells. And therefore she loathes this attribution to the Son of a portion of the one substance; and, because she knows that He was truly born of God, she worships the Son as rightful Possessor of true Divinity. But, for the present, let us defer our full answer to these several allegations, and hasten through the rest of their denunciations.
Source: On the Trinity (New Advent)