39 Jeremiah also, a prophet equally inspired, has taught that God the Only-begotten is of a nature one with that of God the Father. His words are:— This is our God, and there shall be none other likened unto Him, Who has found out all the way of knowledge, and has given it unto Jacob His servant, and to Israel His beloved. Afterward He was seen upon earth, and dwelt among men. Why try to transform the Son of God into a second God? Learn to recognise and to confess the One True God.
No second God is likened to Christ, and so can claim to be God. He is God from God by nature and by birth, for the Source of His Godhead is God. And, again, He is not a second God, for no other is likened unto Him; the truth that is in Him is nothing else than the truth of God. Why link together, in pretended devotion to the unity of God, true and false, base and genuine, unlike and unlike? The Father is God and the Son is God. God is in God; beside Him there is no God, and none other is likened unto Him so as to be God.
If in these Two you shall recognise the Unity, instead of the solitude, of God, you will share the Church's faith, which confesses the Father in the Son. But if, in ignorance of the heavenly mystery, you insist that God is One in order to enforce the doctrine of His isolation, then you are a stranger to the knowledge of God, for you deny that God is in God.
Source: On the Trinity (New Advent)