Against Praxeas by Tertullian
Chapter XXXI.--Retrograde Character of the Heresy of Praxeas The Doctrine of the Blessed Trinity Constitutes the Great Difference Between Judaism and Christianity.
But, (this doctrine of yours bears a likeness) to the Jewish faith, of which this is the substance -- so to believe in One God as to refuse to reckon the Son besides Him, and after the Son the Spirit. Now, what difference would there be between us and them, if there were not this distinction which you are for breaking down? What need would there be of the gospel, which is the substance of the New Covenant, laying down (as it does) that the Law and the Prophets lasted until John the Baptist, if thenceforward the Father, the Son, and the Spirit are not both believed in as Three, and as making One Only God? God was pleased to renew His covenant with man in such a way as that His Unity might be believed in, after a new manner, through the Son and the Spirit, in order that God might now be known openly, in His proper Names and Persons, who in ancient times was not plainly understood, though declared through the Son and the Spirit. Away, then, with those |Antichrists who deny the Father and the Son.| For they deny the Father, when they say that He is the same as the Son; and they deny the Son, when they suppose Him to be the same as the Father, by assigning to Them things which are not Theirs, and taking away from Them things which are Theirs. But |whosoever shall confess that (Jesus) Christ is the Son of God| (not the Father), |God dwelleth in him, and he in God.| We believe not the testimony of God in which He testifies to us of His Son. |He that hath not the Son, hath not life.| And that man has not the Son, who believes Him to be any other than the Son.