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.
Tertullian
Against Praxeas
Chapter XXXI