According to John, when Thomas saw the risen Christ a week after his physical, bodily resurrection he confessed that Jesus was his Lord and God. And instead of rebuking Thomas Jesus actually accepted this exalted confession of faith:
“Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing. And Thomas answered and said unto HIM (eipen AUTO), My Lord and my God (ho Kyrios mou kai ho These mou). Jesus saith unto HIM (legei AUTO), Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed.” John 20:27-29 Authorized King James Version (AV)
In the OT, particularly the Septuagint, the words Lord and God are often associated with each other. In fact, whenever these two words are brought together they always refer to the one true God Jehovah:
“And Israel [has been] a long time without the true God (Theoo alethinoo), and without a priest to expound [the truth], and without the law. But he shall turn them to the Lord God of Israel (Kyrion Theon Israel), and he will be found of them.” 2 Chronicles 15:3-4 LXX
“O Lord my God (Kyrie ho Theos mou), in thee have I trusted: save me from all them that persecute me, and deliver me… O Lord my God (Kyrie ho Theos mou), if I have done this; (if there is unrighteousness in my hands;) if I have requited with evil those who requited me with good; may I then perish empty by means of my enemies.” Psalm 7:1-2, 4-6 [Heb. 1, 3-5]
“O Lord my God (Kyrie ho Theos mou), I cried to thee, and thou didst heal me… O Lord my God (Kyrie ho Theos mou), I will give thanks to thee for ever.” Psalm 29:3, 13 [Heb. 30:2, 12b]
This next example is quite interesting:
“Awake, O Lord (Kyrie), and attend to my judgment, even to my cause, my God and my Lord (ho Theos mou kai ho Kyrios mou). Judge me, O Lord, according to thy righteousness, O Lord my God (Kyrie ho Theos mou); and let them not rejoice against me.” Psalm 34:23-24
The Psalmist’s confession is virtually identical to Thomas’, with the only exception being that the words are reversed:
“In answer Thomas said TO HIM: ‘My Lord and my God (ho Kyrios mou kai ho Theos mou)!’” John 20:28 New World Translation (NWT https://www.jw.org/en/library/bible/nwt/books/john/20/#v43020028)
Thus, Jehovah is the only Lord God that exists for a monotheistic Jew like Thomas, which means that by confessing the risen Jesus to be his very Lord and God Thomas was acknowledging that Jesus is Jehovah in the flesh!
Here are some Evangelical scholars that help us understand just how truly significant this confession is seeing that it comes from the lips of a monotheistic Jew:
“On occasion Thomas’s statement has been interpreted as an exclamation that expresses his praise to God for the miracle of the resurrection of Jesus: ‘Praise be to my Lord and my God!’ Fatal to this interpretation is the phrase said to him (i.e. Jesus) (eipen auto), which is clearly parallel to the surrounding verses: ‘He [Jesus] said to Thomas’ (v. 27) and ‘Jesus said to him [Thomas]’ (v. 29). What we have in verse 28 is not an ejaculation made in the hearing of Jesus but an exclamation actually addressed to him. In effect Thomas is saying, ‘You are my Lord and my God.’ He recognized that Jesus, now alive from the dead, was supreme over all physical and spiritual life (‘Lord’) and one who shared the divine nature (‘God’).
“Was Thomas’s cry an extravagant acclamation, spoken in a moment of ecstasy when his exuberance outstripped his theological sense? Not at all. John records no rebuke of Jesus to Thomas for his worship. Jesus’ silence is tantamount to consent, for Jews regarded the human acceptance of worship as blasphemous. Indeed, Jesus’ subsequent word to Thomas, ‘you have believed’ (v. 29 a), implies that he accepted Thomas’s confession of faith, which he then indirectly commends to others (v. 29b). Moreover, John himself has endorsed Thomas’s confession, for it stands as his last and highest affirmation about Christ immediately before his statement of purpose in writing the Gospel (vv. 30-31).” (Murray J. Harris, 3 Crucial Questions about Jesus [Baker Books, Grand Rapids, MI 1994], pp. 94-95; bold emphasis ours)
“There is essentially no controversy among biblical scholars that in John 20:28 Thomas is referring to and addressing Jesus when he says, ‘My Lord and my God!’ As Harris says in his lengthy study on Jesus as God in the New Testament, ‘This view prevails among grammarians, lexicographers, commentators and English versions.’ Indeed, it is difficult to find any contemporary exegetical commentary or academic study that argues that Thomas’s words in John 20:28 apply in context to the Father rather than to Jesus. The reason is simple: John prefaces what Thomas said with the words, ‘Thomas answered and said to him’ (v. 28a NASB). This seemingly redundant wording reflects a Hebrew idiomatic way of introducing someone’s response to the previous speaker. John uses it especially frequently, always with the speaker’s words directed to the person or persons who have just spoken previously in the narrative (John 1:48, 50; 2:18-19; 3:3, 9-10, 27; 4:10, 13, 17; 5:11; 6:26, 29, 43; 7:16, 21, 52; 8:14, 39, 48; 9:11, 20, 30, 34, 36; 12:30; 13:7; 14:23; 18:30; 20:28). It is therefore certain that Thomas was directing his words to Jesus, not to the Father. No one, of course, would ever have questioned this obvious conclusion if Thomas had said simply ‘My Lord!’ It is the addition of the words ‘and my God’ that have sparked some creative but untenable interpretations of the text.
“Thomas’s words echo statements addressed in the Psalms to the Lord (Jehovah), especially: ‘Wake up!’ Bestir yourself for my defense, for my cause, my God and my Lord [ho theos mou kai ho kurios mou]!’ (Ps. 35:23). These words parallel those in John 20:28 exactly except for reversing ‘God’ and ‘Lord’. More broadly, in biblical language ‘my God’ (on the lips of a faithful believer) can refer only to the Lord God of Israel. The language is as definite as it could be and identifies Jesus Christ as God himself.
“In identifying Jesus as God, Thomas, of course, was not identifying him as the Father. Earlier in the same passage, Jesus had referred to the Father as his God. It is interesting to compare Jesus’ wording with the wording of Thomas. Jesus told Mary Magdalene, ‘I am ascending to my Father and your Father, to my God and to your God’ (theon mou kai theon humon, John 20:17). As in John 1:1 and John 1:18, the Father is called ‘God’ in close proximity to a statement affirming that Jesus is also ‘God.’ Here again, as in John 1:18, we do not see the apostle John distinguishing between the Father as ‘the God’ (ho theos) and Jesus the Son as only ‘God’ (theos without the article). In fact, whereas Jesus calls the Father ‘my God’ without the article (theon mou, 20:17), Thomas calls Jesus ‘my God’ with the article (ho theos mou, 20:28)! One could not ask for any clearer evidence that the use or nonuse of the article is irrelevant to the meaning of the word theos. What matters is how the word is used in context. In John 20:28, the apostle reports the most skeptical of disciples making the most exalted of confessions about Jesus, John expects his readers to view Thomas’s confession as a model to follow. Recognizing Jesus as the One who has conquered death itself for us, we too are to respond to Jesus and confess that he is our Lord and God.” (Putting Jesus in His Place: The Case for the Deity of Christ, Chapter 12. Immanuel: God with Us, pp. 142-143; bold emphasis ours)
Even anti-Trinitarians like the Jehovah’s Witnesses admit that when an Israelite says “my God” he can only be referring to Jehovah his God:
“In its articles on JEHOVAH, the Imperial Bible Dictionary (Vol. I, p. 856) nicely illustrates the difference between Elohim (God) and Jehovah. Of the name Jehovah, it says: ‘It is everywhere a proper name, denoting the personal God and him only; whereas Elohim partakes more of the character of a common noun, denoting usually, indeed, but not necessarily nor uniformly, the Supreme…. The Hebrew may say theElohim, the true God, in opposition to all false gods; but he never says the Jehovah, for Jehovah is the name for the true God only. He says again and again my God…; but never my Jehovah, for when he says my God, He means Jehovah. He speaks of the God of Israel, but never of Jehovah of Israel, for there is no other Jehovah. He speaks of the living God, but never of the living Jehovah, for he cannot conceive of Jehovah as other than living.’” (Aid to Bible Understanding [Watch Tower Bible and Tract Society of New York, Inc., 1971], p. 885; bold emphasis ours)
To repeat the significance of Thomas’ confessing Jesus as his Lord God. Since the only Lord God that a monotheistic Jew can ever confess is Jehovah, seeing that he is the only true God that exists, this means that Thomas was proclaiming that Jesus is none other Jehovah Almighty in the flesh.
And since Jesus accepted Thomas’ confession this shows that Jesus must have believed himself to the only true God Jehovah.
Finally, since Jesus affirmed that the Father is the only true God and since he is not the Father, this means that Jesus wasn’t a Unitarian nor was he a Muslim. Rather, the evidence conclusively proves that Jesus confirmed and taught his followers that the only true God was/is multi-personal and that he was also the only true God in union with the Father (as well as the Holy Spirit).