35 The distorted mind of heresy is incapable of knowing and confessing the One true God; the sound faith and reason necessary for such confession is incompatible with unbelief. We must confess Father and Son before we can apprehend God as One and true. When we have known the mysteries of man's salvation, accomplished in us through the power of regeneration unto life in the Father and the Son, then we may hope to penetrate the mysteries of the Law and the Prophets. Godless ignorance of the teaching of Evangelists and Apostles cannot frame the thought of One true God.
Out of the teaching of Evangelists and Apostles we shall present the sound doctrine concerning Him, in accurate agreement with the faith of true believers. We shall present Him in such wise that the Only-begotten, Who is of the substance of the Father, shall be known as indivisible and inseparable in nature, not in Person. We shall set forth God as One, because God is from the nature of God. But we shall also establish this doctrine of the perfect unity of God upon the words of the Prophets, and make them the foundations of the Gospel structure, proving that there is One God, with one Divine nature, by the fact that God the Only-begotten is never classed apart as a second God.
For throughout this book of our treatise we have followed the same course as in its predecessor; the same methods which proved there that the Son is God, have proved here that He is true God. I trust that our explanation of each passage has been so convincing that we have now manifested Him as true God as effectually as we formerly demonstrated His Godhead. The remainder of the book shall be devoted to the proof that He, Who is now recognised as true God, must not be regarded as a second God. Our disproof of the notion of a second God will further establish the unity; and this truth shall be displayed as not inconsistent with the personal existence of the Son, while yet it maintains the unity of nature in God and God.
Source: On the Trinity (New Advent)