What does John 20:28 mean?

Thomas had stubbornly refused to believe Jesus was resurrected, despite the eyewitness accounts of Jesus' closest followers (John 20:17–23). That resulted in an outrageous demand: to put his fingers into Jesus' wounds before accepting that He was alive (John 20:24–25). When Jesus appeared, again, He confronted Thomas with that challenge (John 20:26–27). As part of that, Jesus noted that Thomas was not merely failing to accept truth: he was actively dis-believing.

Skepticism, to a point, is a good thing (Acts 17:11; 1 John 4:1; 2 Corinthians 13:5). What's not good is pretending to want evidence while demanding more evidence than is necessary—or even possible. When Thomas made his ultimatum, he probably assumed there could never be any scenario where that would even be possible. There's a parallel in modern "skepticism," which often sets exorbitant demands for God to "prove" Himself…while ignoring available evidence and having no intent of believing, anyway (John 5:39–40).

Thomas may have touched Jesus' wounds before he believed, but the Bible does not explicitly say so. What seems more likely is that he responded to Jesus' appearance—and scolding—with this declaration of faith. Jesus won't refer to Thomas responding to touch, but only to what he has now seen.

Incredible meaning is attached to what Thomas says here: he not only declares Jesus his Master, but he overtly refers to Jesus as "my God!" If Thomas is saying something wrong, this is an opportunity for Jesus to correct him. Other spiritual beings do exactly that when someone attempts to worship them (Revelation 19:10; 22:8–9). Jesus does not dispute this title, because He is God (John 5:18; 8:58; 10:30). His next remarks gently reprimand Thomas for insisting on excessive proof. He will point out that many people will not have the opportunity which the disciples were given (John 20:29)