The Confession of St. Thomas the Apostle
And now let us see whether the confession of Thomas the Apostle, when he cried, "My Lord and My God," corresponds with this assertion of the Evangelist. We see that he speaks of Him, Whom he confesses to be God, as My God. Now Thomas was undoubtedly familiar with those words of the Lord, "Hear, O Israel, the Lord your God is One." How then could the faith of an Apostle become so oblivious of that primary command as to confess Christ as God, when life is conditional upon the confession of the Divine unity?
It was because, in the light of the Resurrection, the whole mystery of the faith had become visible to the Apostle. He had often heard such words as, "I and the Father are One, and, All things that the Father has are Mine," and, "I in the Father and the Father in Me;" and now he can confess that the name of God expresses the nature of Christ, without peril to the faith. Without breach of loyalty to the One God, the Father, his devotion could now regard the Son of God as God, since he believed that everything contained in the nature of the Son was truly of the same nature with the Father. He no longer need to fear that such a confession was the proclamation of a second God, a treason against the unity of the Divine nature–for it was not a second God Whom that perfect birth of the Godhead had brought into being. Thus it was with full knowledge of the mystery of the Gospel that Thomas confessed his Lord and his God. It was not a title of honor; it was a confession of nature. He believed that Christ was God in substance and in power.
The Lord, in turn, shows that this act of worship was the expression not of mere reverence, but of faith, when He says, "You have believed because you have seen Me. Blessed are those who have not yet seen and yet have believed." Thus, Thomas had seen before he believed.
– On the Trinity, Book 7, NPNF s. 1, v. 7.