Thomas is kinda funny to. The verse does not say Jesus is God nor is it a teaching that Jesus is God. “My Lord and my God.” A very likely way to understand John 20:28 is that Thomas had realized the power of God working in Jesus, and in saying “my Lord and my God” he was pointing out that Jesus did reveal God in a unique and powerful way. In seeing the resurrected Jesus, Thomas clearly saw both the Lord Jesus, and the God who raised Jesus from the dead. Jesus always taught that he only did what God guided him to do, and said that if you had seen him you had seen the Father. In that light, there is good evidence that “doubting Thomas” was saying that in seeing Jesus he was also seeing the Father.
We have to remember that Thomas’ statement occurred in a moment of surprise and even perhaps shock. Only eight days earlier, Thomas had vehemently denied Jesus’ resurrection. Thomas could no longer deny that Jesus was alive and that God had raised him from the dead. Thomas, looking at the living Jesus, saw both Jesus and the God who raised him from the dead. When Thomas saw the resurrected Christ, he became immediately convinced that Jesus was raised from the dead. But did he suddenly have a revelation that Jesus was God? That would be totally outside of Thomas’ knowledge and belief. Jesus had never claimed to be God despite Trinitarian claims that he had.
In other places in the Bible where the apostles speak about the resurrection of Jesus, they do not declare “This proves Jesus is God!” Rather, they declare that God raised the Lord Jesus from the dead. The confession of the two disciples walking along the road to Emmaus demonstrated the thoughts of Jesus’ followers at the time. Speaking to the resurrected Christ, whom they mistook as just a traveler, they talked about Jesus. They said Jesus “was a prophet mighty in deed and word before God and they crucified him." The disciples thought Jesus was the Messiah, a “Prophet” and the Son of God, but not God Himself.
Are we to believe that somehow Jesus taught the Trinity, something that went against everything the disciples were taught and believed, but there is no mention of Jesus ever teaching it anywhere, and yet the disciples somehow got that teaching? That seems too incredible to believe. There is no evidence from the gospel accounts that Jesus’ disciples believed him to be God, and Thomas upon seeing the resurrected Christ was not birthing a new theology in a moment of surprise.