What does John 20:29 mean?

Jesus' disciples were resistant to the idea that He had been resurrected. They had heard Him predict that event (John 2:19–22; Luke 18:33). They'd seem Him raise others from death (John 11:43–44). Eyewitnesses had seen Him alive (John 20:17–18; Luke 24:10–11; 23–24). Still, it wasn't until Jesus personally appeared to them that they accepted the truth (John 20:19–23). Thomas had been especially stubborn, but also came to believe once Jesus appeared to him (John 20:24–28).

The comment Jesus makes here points out that most people will never have the experiences which these men enjoyed. As the disciples go out to preach the gospel, they will be speaking to people who will not see Jesus in person. They will preach to those without the benefit of direct, personal encounters with God. That should be humbling—these men will never have cause to think they're somehow better, or more spiritual, than those to whom they preach. Jesus does not mean that it is better to believe without hard proof (Acts 17:11; 1 John 4:1; John 5:39–40). He simply notes that not everyone is blessed with the same level of evidence.

The same humility ought to apply for believing persons today. Though some of God's truth is obvious (Romans 1:18–20; Psalm 19:1), modern advances have given us resources far beyond those of prior generations. Evidence, texts, arguments, and examples which scholars of the past never saw are available to almost every person today. Before we pat ourselves on the back for belief, or our response to the gospel, we should remember statements like this from Christ. Those alive today have truly "seen" things which add weight to the truths God wants us to understand (Hebrews 12:1–2).

John's aside, in the next verses (John 20:30–31) follows from that same idea. That which God has given us ought to be enough (John 20:30–31) and was chosen for a reason (John 21:25).