What does John 20:27 mean?
Many skeptics of religion, especially of Christianity, demand proof far beyond what's reasonable. Scripture approves of cautious skepticism (Acts 17:11; 1 John 4:1; 2 Corinthians 13:5). It does not indulge those who are simply trying to excuse their choice not to believe. A common result of this is known as "moving the goal posts;" this expression means demanding a level of certainty, only to re-define or re-locate those requirements after the original demand is met. In this way, no matter what happens, the person can continue to claim there is not enough evidence.In Thomas' case, Jesus had predicted His own resurrection (John 2:19–22). Thomas had seen Him bring people out of death (John 11:43–44). Eyewitnesses had seen Jesus, in person, alive after His crucifixion (John 20:17–18; Luke 24:10–11; 23–24). Christ had even appeared in a locked room, speaking to Thomas' fellow disciples (John 20:19–24). Rather than accept the obvious, Thomas made what he likely thought was an absurd, impossible demand: to put his own fingers into Jesus' crucifixion wounds before believing in the resurrection (John 20:25).
When Jesus reappears, again in a locked room, He challenges Thomas to make good on his claim. Jesus is now there, in the flesh, offering Thomas the opportunity to do what he said was needed to make him believe. Jesus' comment here indicates that Thomas' choice is exactly that: a decision not to accept what he ought to believe. This is a common theme among those who reject faith in Christ (John 5:39–40). Thomas has not been sincere, he has been obstinate. The question he is now faced with is whether he will continue to "move the goal posts," or admit the truth.
Scripture does not say, explicitly, whether Thomas literally did this or not. The flow of John's account makes it seem he did not. Rather, he appears to immediately respond to Jesus' mere presence. Whether Thomas touched Jesus' wounds, or not, his response is—finally—the correct one. He will accept the truth and acknowledge Jesus for who He is (John 20:28).