Jesus is God in John 20:28
Thomas answered him, “My Lord and my God!” (John 20:28 NRSV)
Grammatically speaking, Jesus is being addressed as both Lord and God by Thomas in John 20:28.
(1) ho kurios mou kai o theos mou (the Lord of me and the God of me) is an unsual Greek grammatical structure known as ''Hebraism''. In this syntax, both the nominatives o kurios (the Lord) and o theos (the God) refers to the same person being addressed.The exact same Hebraism is found in Psalm 35:23 of the Greek Septuagint with only the nominatives in juxtaposition here (ho theos mou kai ho kyrios mou - the God of me and the Lord of me).
(2) Jesus is explicitly the only one being addressed in John 20:28 as evident in another Hebraism ''and he answered and said unto him'' [autos]. The antecedent of the pronoun here refers to Iesous (Jesus) alone.
John 14:9 says that seeing Jesus is seeing the Father (see also John 12:45).
This is entirely different from saying ''my God'' to Jesus in John 20:28. Thomas was not addressing the Father in John 20:28. Thomas did not say ''my God'' to the Father in John 20:28. The author explicitly writes the pronoun αὐτῷ which in context has Jesus (not the Father) as the antecedent.
Jesus is God in John 20:28 based on context:
(1) John 20:22 supports John 20:28. In John 20:22, John applies to Jesus the same Greek word as well as the same action YHWH did in Genesis 2:7 (LXX) to Jesus.
Jesus is the same God who breathed on man in Genesis 2:7 as evident in the Greek texts:
The Greek word ἐνεφύσησεν (breathed on) occurred only once in the Old Testament in Genesis 2:7. In this verse, ὁ θεὸς (the God) breathed on [the nostrils of the man].
The Greek word ἐνεφύσησεν (breathed on) also occurred only once in the New Testament in John 20:22. In this chapter, the one who breathes on [the disciples] is also ὁ θεὸς (the God) in John 20:28 but he isn't the Father but Jesus Christ. Grammatically, John 20:28 calls Jesus ὁ θεὸς (the God) and this is strengthen by the context wherein Jesus ἐνεφύσησεν (breathed on) his disciples.
Only ὁ θεὸς (the God) ἐνεφύσησεν (breathed on) persons in the Old Testament (Genesis 2:7). In the New Testament, Jesus is ὁ θεὸς (the God) who also ἐνεφύσησεν (breathed on) persons (John 20:22, 20:28).
Whether we interpret God breathing on man either figuratively or not, the fact remains that the one who did the breathing on people is just the same O ΘΕΟΣ (the God) which the texts identify as Jesus Christ (John 20:22, 20:28).
I am taking excerpts from my article ''Jesus as θεός in John 20:28: An Inductive Analysis and Exploratory Research'' (2020). I hope it helps:
''This study investigated the unusual Greek of John 20:28 in which two articular substantives have a singular referent. This study searched through the Septuagint, the Greek New Testament canon, Biblical scholars and Greek grammarians. This study analysed data from an exploratory and inductive analyses to form new hypotheses from data. In the analyses, the researcher reports that Hebraism is the reason for the unusual Greek of John 20:28. The following are the findings in this study which explains why the two articular substantives (Ὁ κύριός μου καὶ ὁ θεός μου) in John 20:28 is referring to two persons in Koine Greek but has only one person in view: (1) The New Testament writers, acquainted with the Septuagint, followed after its usage of ὁ κύριος (The Lord) when referring to the God of Israel. Psalm 35:23, Revelation 4:11 and John 20:28 all have ὁ κύριος (The Lord). This indicates that both titles in John 20:28 are equally divine titles (2) the dative singular pronoun (αὐτῷ) in the same text is addressing a singular referent (Ἰησοῦς), and this shows that the substantives are in the vocatical nominative (direct address) and that this is in agreement with that of Sharp (1803) and Murray Harris's results (2009). The Greek of John 20:28 is not unusual but normative in the Septuagint. (2) It is shown that the Greek of John 20:28 is consistent with its parallels in the Septuagint, showing that its Greek is an example of Hebraism. There are three instances of Hebraism in the gospel of John (John 4:12, John 12:14, and John 28:28). Psalm 35:23 is not only a parallel but even an allusion to John 20:28 which supports its being a Hebraism. An LXX expert, Solamo (2015) had explicitly affirmed that John 20:28 is a Hebraism. Contextual analysis within the same chapter of John 20 supports these results even further due to another allusion from the LXX (Genesis 2:7, John 20:22) wherein Jesus was ascribed an attribute unique to the Jewish deity with a very supportive ὁ θεός allusion for 20:28. This finding provided objective evidence that Jesus is θεός in John 20:28.''
