Simple doubt is honest.  It says that we do not know or cannot be sure, but it is open to instruction and help (Mark 9:24).  However, Thomas’s doubt was not like that.  It was wilful, aggressive and hard-hearted (Mark 16:14).  Despite the evidence of the other disciples and the reports from the women who had been at the tomb, Thomas was determined not to believe unless he had physical first-hand evidence.  His doubt had extended to an arrogance which called the other disciples liars and ignored Christ's promise of resurrection (Matthew 20:17-19).
 
The following Sunday, Jesus appeared to them again.  Once again, the doors were locked but Jesus appeared in the room without invitation or any hinderance (John 20:19), extending His peace.  He had already heard Thomas’s protests and now singled him out, inviting him to touch His wounded body and feel where the nails and spear had gone.  It must have been an awesome moment for Thomas.

But linked with that invitation was a clear rebuke to stop doubting and believe.  There is no record that Thomas did put his finger into the nail marks, but he did believe.  His exclamation that Jesus was both Lord and God showed his faith and marked his repentance (John 20:28).
 
Although some people are timid in their doubt, and others appear not to care when they are ignorant, aggressive doubt about Jesus is a sign of spiritual rebellion.  But those of our friends who behave in that way are probably far more engaged with the gospel than they may realise.  Their engagement is not positive, dismissing His grace as irrelevant or claiming to have more evidence about Jesus.  But it is negative fighting talk.  When that happens, be encouraged.  A power struggle is under way.  Their pride keeps them fighting until they hear the Lord’s rebuke.  The sign of their submission is that they acknowledge Jesus as Lord and God.  So, keep praying and witnessing!