9 “For He whom God has sent speaks the words of God.” Himself is the true God, and God sent Him: God sent God. Join both, one God, true God sent by God. Ask concerning them singly, He is God; ask concerning them both, they are God. Not individually God, and both Gods; but each individual God, and both God. For so great is the charity of the Holy Spirit there, so great the peace of unity, that when you question about them individually, the answer to you is, God; when you ask concerning the Trinity, you get for answer, God.
For if the spirit of man, when it cleaves to God, is one spirit, as the apostle openly declares, “He that is joined to the Lord is one spirit;” how much more is the equal Son, joined to the Father, together with Him one God! Hear another testimony. You know how many believed, when they sold all they had and laid it at the apostles' feet, that it might be distributed to each according to his need; and what says the Scripture of that gathering of the saints? “They had one soul and one heart in the Lord.” If charity made one soul of so many souls, and one heart of so many hearts, how great must be the charity between the Father and the Son!
Surely it must be greater than that between those men who had one heart. If, then, the heart of many brethren was one by charity, if the soul of many brethren was one by charity, would you say that God the Father and God the Son are two? If they are two Gods, there is not the highest charity between them. For if charity is here so great as to make your soul and your friend's soul one soul, how can it be then that the Father and the Son is not one God? Far be unfeigned faith from this thought.
In short, how excellent that charity is, understand hence: the souls of many men are many, and if they love one another, it is one soul; still, in the case of men, they may be called many souls, because the union is not so strong. But there it is right for you to say one God; two or three Gods it is not right for you to say. From this, the supreme and surpassing excellency of charity is shown you to be such, that a greater cannot be.
Source: Tractates on the Gospel of John (New Advent)