25 And now the time has come for us to put a stop to that cunning artifice of heresy, by which they pervert the devout and godly teachings of the Law into a support for their own godless delusion. They preface their denial of the Son of God with the words, Hear, O Israel, the Lord your God is One; and then, because their blasphemy would be refuted by the identity of name, since the Law speaks of God and God, they invoke the authority of the prophetic words, They shall bless You, the true God, to prove that the name is not used in the true sense.
They argue that these words teach that God is One, and that God, the Son of God, has His name only and not His nature; and that therefore we must conclude that the true God is one Person only. But perhaps you imagine, fool, that we shall contradict these texts of yours, and so deny that there is one true God. Assuredly we do not contradict them by a confession conceived in your sense. Our faith receives them, our reason accepts them, our words declare them. We recognise One God, and Him true God.
The name of God has no dangers for our confession, which proclaims that in the nature of the Son there is the One true God. Learn the meaning of your own words, recognise the One true God, and then you will be able to make a faithful confession of God, One and true. It is the words of our faith which you are turning into the instrument of your blasphemy, preserving the sound and perverting the sense. Masquerading in a foolish garb of imaginary wisdom, under cover of loyalty to truth you are the truth's destroyer.
You confess that God is One and true, on purpose to deny the truth which you confess. Your language claims a reputation for piety on the strength of its impiety, for truth on the strength of its falsehood. Your preaching of One true God leads up to a denial of Him. For you deny that the Son is true God, though you admit that He is God, but God in name only, not in nature. If His birth be in name, not in nature, then you are justified in denying His true right to the name; but if He be truly born as God, how then can He fail to be true God by virtue of His birth?
Deny the fact, and you may deny the consequence; if you admit the fact, how can He be other than Himself? No being can alter its own essential nature. About His birth I shall speak presently; meantime I will refute your blasphemous falsehoods concerning His true Divine nature by the utterances of prophets. But I shall take care that in our assertion of the One true God I give no cover to the Sabellian heresy that the Father is one Person with the Son, and none to that slander against the Son's true Godhead, which you evolve out of the unity of the One true God.
Source: On the Trinity (New Advent)