12 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.
No longer need he fear that such a confession as his 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 honour; it was a confession of nature. He believed that Christ was God in substance and in power. And the Lord, in turn, shows that this act of worship was the expression not of mere reverence, but of faith, when He says, Because you have seen, you have believed; blessed are they which have not seen, and have believed.
For Thomas had seen before he believed. But, you ask, What was it that Thomas believed? That, beyond a doubt, which is expressed in his words, My Lord and my God. No nature but that of God could have risen by its own might from death to life; and it is this fact, that Christ is God, which was confessed by Thomas with the confidence of an assured faith. Shall we, then, dream that His name of God is not a substantial reality, when that name has been proclaimed by a faith based upon certain evidence?
Surely a Son devoted to His Father, One Who did not His own will but the will of Him that sent Him, Who sought not His own glory but the glory of Him from Whom He came, would have rejected the adoration involved in such a name as destructive of that unity of God which had been the burden of His teaching. Yet, in fact, He confirms this assertion of the mysterious truth, made by the believing Apostle; He accepts as His own the name which belongs to the nature of the Father. And He teaches that they are blessed who, though they have not seen Him rise from the dead, yet have believed, on the assurance of the Resurrection, that He is God.
Source: On the Trinity (New Advent)