26 Next the seventh book, starting from the basis of a true faith now attained, delivers its verdict in the great debate. First, armed with its sound and incontrovertible proof of the impregnable faith, it takes part in the conflict raging between Sabellius and Hebion and these opponents of the true Godhead. It joins issue with Sabellius on his denial of the pre-existence of Christ, and with his assailants on their assertion that He is a creature. Sabellius overlooked the eternity of the Son, but believed that true God worked in a human body.
Our present adversaries deny that He was born, assert that He was created, and fail to see in His deeds the works of very God. What both sides dispute, we believe. Sabellius denies that it was the Son who was working, and he is wrong; but he proves his case triumphantly when he alleges that the work done was that of true God. The Church shares his victory over those who deny that in Christ was very God. But when Sabellius denies that Christ existed before the worlds, his adversaries prove to conviction that Christ's activity is from everlasting, and we are on their side in this confutation of Sabellius, who recognises true God, but not God the Son, in this activity.
And our two previous adversaries join forces to refute Hebion, the second demonstrating the eternal existence of Christ, while the first proves that His work is that of very God. Thus the heretics overthrow one another, while the Church, as against Sabellius, against those who call Christ a creature, against Hebion, bears witness that the Lord Jesus Christ is very God of very God, born before the worlds and born in after times as man.
Source: On the Trinity (New Advent)