Jesus had met His disciples on the Sunday evening when He rose from the dead (John 20:19).  But Thomas was not there and later refused to believe that Jesus was alive unless he had physical evidence (John 20:24-25).  The following Sunday, Jesus joined the disciples in the same room, where the doors were still locked because they were still afraid of the Jewish leaders who might arrest them.  This time, Thomas was there.

Jesus confronted the doubting disciple with His wounded hands and side, rebuking him for his faithlessness.  Exposing the nail prints in his hands, the evidence was so overwhelming that Thomas had no option but to admit that Jesus was alive (John 20:26-27).  More than that, he called Jesus, “My Lord and my God” - God to be worshipped and Lord to be obeyed (John 20:28).  Jesus did not rebuke Thomas for his worship.  Calling Jesus ‘his God’ was not blasphemous; it was the truth.  Thomas’ refusal to believe was defeated; he believed because of what he saw.
 
But Jesus did not agree that Thomas’ example of demanding a physical sign should be the normal route to faith.  Instead, Jesus taught that faith should be based on believing the truth about Himself, from those who had seen Him.  Thomas had failed in that, by disregarding what the other disciples had said.  Jesus said that many people would believe in Him, although they could never see Him physically, because they trusted what eye-witnesses said about Him in the gospel (John 17:20).  Indeed, the whole future of the church would depend on people believing what the Apostles said about Jesus (Acts 2:42).  That kind of believing is real faith which God will always reward and bless.  We have Jesus’ promise!
 
But many people in the world today have never heard the eye-witness evidence of the Apostles who saw Jesus after His resurrection.  They do not know His character or why He came to earth, died and rose again.  Wonderfully, all they need to know about Jesus has already been written for us in the Bible - in particular, what the Apostles recorded in the New Testament.  No other information is needed.  And every person who believes in Jesus is to pass on that information to a Christless world.  There is no need for God to prove Himself in some new way.  All we need to do is to believe in Jesus through what is already written (1 Peter 1:23).  When our friends do that, they will be saved and become children of God.  So, telling other people about Jesus is the most important thing we can do for them.  As Romans 10:17 says, “… faith comes from hearing the message ...”