Deuteronomy 6:4

That Trinity is one God. Not that Father, Son and Holy Spirit are identically the same. But the Father is Father, the Son is Son, and the Holy Spirit is Holy Spirit, and this Trinity is one God, as it is written: "Hear, O Israel, the Lord your God is one God."

Consider now for a while the passages of Scripture which force us to confess that the Lord is one God, whether we are asked about the Father alone, or the Son alone, or the Holy Spirit alone, or about the Father and the Son and the Holy Spirit together. Certainly it is written, "Hear, O Israel, the Lord your God is one Lord." Of whom do you think that this is said? If it is said only of the Father, then our Lord Jesus Christ is not God. Why did those words come to Thomas when he touched Christ and cried out, "My Lord and my God," which Christ did not reprove but approved, saying, "Because you have seen, you have believed"?

This is the cause why it says, "the Lord our God is one Lord," and also proclaims the Only-begotten God by the name of Godhead, without dividing the Unity into a dual signification, so as to call the Father and the Son two Gods, although each is proclaimed by the holy writers as God. The Father is God: the Son is God: and yet by the same proclamation God is One, because no difference either of nature or of operation is contemplated in the Godhead. For if (according to the idea of those who have been led astray) the nature of the Holy Trinity were diverse, the number would by consequence be extended to a plurality of Gods, being divided according to the diversity of essence in the subjects. But since the Divine, single, and unchanging nature, that it may be one, rejects all diversity in essence, it does not admit in its own case the signification of multitude; but as it is called one nature, so it is called in the singular by all its other names, "God," "Good," "Holy," "Saviour," "Just," "Judge," and every other Divine name conceivable.

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 thy 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 hath 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.

Therefore, in whatever place you may be, because you know that you have been baptized in the one name of the Father and the Son and the Holy Spirit, according to the rule promulgated by the command of our Savior, retain this rule with your …