11 What follows is this:— “Nor, as Sabellius, who makes two out of One, Son and Father at once.” Sabellius holds this in wilful blindness to the revelation of the Evangelists and Apostles. But what we see here is not one heretic honestly denouncing another. It is the wish to leave no point of union between Father and Son that prompts them to reproach Sabellius with his division of an indivisible Person; a division which does not result in the birth of a second Person, but cuts the One Person into two parts, one of which enters the Virgin's womb. But we confess a birth; we reject this confusion of two Persons in One, while yet we cleave to the Divine unity.
That is, we hold that God from God means unity of nature; for that Being, Who, by a true birth from God, became God, can draw His substance from no other source than the Divine. And since He continues to draw His being, as He drew it at first, from God, He must remain true God for ever; and hence They Two are One, for He, Who is God from God, has no other than the Divine nature, and no other than the Divine origin. But the reason why this blasphemous Sabellian confusion of two Persons into One is here condemned is that they wish to rob the Church of her true faith in Two Persons in One God.
But now I must examine the remaining instances of this perverted ingenuity, to save myself from the reputation of a censorious judge of sincere enquirers, moved rather by dislike than genuine fear. I shall show, by the terms with which they wind up their confession, what is the deadly conclusion which they have skilfully contrived shall be its inevitable issue.
Source: On the Trinity (New Advent)