Albert Barnes Commentary John 20:28

"Thomas answered and said unto him, My Lord and my God." — John 20:28 (ASV)

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— "Thomas— said 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. (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 profanity, 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.