My Lord and my God - In this passage the name God is expressly given to Christ, in his own presence and by one of his own apostles. This declaration has been considered as a clear proof of the divinity of Christ, for the following reasons:
1. There is no evidence that this was a mere expression, as some have supposed, of surprise or astonishment.
2. The language was addressed to Jesus himself - "Thomassaid unto him."
3. The Saviour did not reprove him or check him as using any improper language. If he had not been divine, it is impossible to reconcile it with his honesty that he did not rebuke the disciple. No pious man would have allowed such language to be addressed to him. Compare Acts 14:13-15; Revelation 22:8-9.
4. The Saviour proceeds immediately to commend Thomas for believing; but what was the evidence of his believing? It was this declaration, and this only. If this was a mere exclamation of surprise, what proof was it that Thomas believed? Before this he doubted. Now he believed, and gave utterance to his belief, that Jesus was his Lord and his God.
5. If this was not the meaning of Thomas, then his exclamation was a mere act of profaneness, and the Saviour would not have commended him for taking the name of the Lord his God in vain. The passage proves, therefore, that it is proper to apply to Christ the name Lord and God, and thus accords with what John affirmed in John 1:1, and which is established throughout this gospel.

Thomas answered, etc. - Those who deny the Godhead of Christ would have us to believe that these words are an exclamation of Thomas, made through surprise, and that they were addressed to the Father and not to Christ. Theodore of Mopsuestia was the first, I believe, who gave the words this turn; and the fifth Ecumenic council, held at Constantinople, anathematized him for it. This was not according to the spirit of the Gospel of God. However, a man must do violence to every rule of construction who can apply the address here to any but Christ. The text is plain: Jesus comes in - sees Thomas, and addresses him; desiring him to come to him, and put his finger into the print of the nails, etc. Thomas, perfectly satisfied of the reality of our Lord's resurrection, says unto him, - My Lord! and My God! i.e. Thou art indeed the very same person, - my Lord whose disciple I have so long been; and thou art my God, henceforth the object of my religious adoration. Thomas was the first who gave the title of God to Jesus; and, by this glorious confession, made some amends for his former obstinate incredulity. It is worthy of remark, that from this time forward the whole of the disciples treated our Lord with the most supreme respect, never using that familiarity towards him which they had often used before. The resurrection from the dead gave them the fullest proof of the divinity of Christ. And this, indeed, is the use which St. John makes of this manifestation of Christ. See John 20:30, John 20:31. Bishop Pearce says here: "Observe that Thomas calls Jesus his God, and that Jesus does not reprove him for it, though probably it was the first time he was called so." And, I would ask, could Jesus be jealous of the honor of the true God - could he be a prophet - could he be even an honest man, to permit his disciple to indulge in a mistake so monstrous and destructive, if it had been one?

And Thomas answered and said unto him,.... Without examining his hands and side, and as astonished at his condescension and grace, and ashamed of his unbelief:
my Lord and my God; he owns him to be Lord, as he was both by creation and redemption; and God, of which he was fully assured from his omniscience, which he had given a full proof of, and from the power that went along with his words to his heart, and from a full conviction he now had of his resurrection from the dead. He asserts his interest in him as his Lord and his God; which denotes his subjection to him, his affection for him, and faith in him; so the divine word is called in Philo the Jew, , "my Lord" (x).
(x) Lib. Allegor. l. 2. p. 101.

Thomas answered and said unto him, My Lord and my God--That Thomas did not do what Jesus invited him to do, and what he had made the condition of his believing, seems plain from John 20:29 ("Because thou hast seen Me, thou hast believed"). He is overpowered, and the glory of Christ now breaks upon him in a flood. His exclamation surpasses all that had been yet uttered, nor can it be surpassed by anything that ever will be uttered in earth or heaven. On the striking parallel in Nathanael, see on John 1:49. The Socinian invasion of the supreme divinity of Christ here manifestly taught--as if it were a mere call upon God in a fit of astonishment--is beneath notice, save for the profanity it charges upon this disciple, and the straits to which it shows themselves reduced.

And Thomas said, My Lord and my God - The disciples had said, We have seen the Lord. Thomas now not only acknowledges him to be the Lord, as he had done before, and to be risen, as his fellow disciples had affirmed, but also confesses his Godhead, and that more explicitly than any other had yet done. And all this he did without putting his hand upon his side.