We know Thomas had doubted but his doubts were put to rest by Jesus himself.  Verse 17 probably refers to the larger group of disciples.  Even though Jesus was appearing to them in the flesh some still had a hard time believing it was possible that Jesus had rose again.  They were thinking carnally not spiritually.  We as believers know all things are possible with God.  Even today among people who claim to be Christian there are some who don’t believe that Jesus rose from the dead.  They can’t understand the Jesus that claim to follow if they can’t believe Jesus rose again.  Jesus was the Son of God death had no power of him and because he rose again we know we shall also rise again and joined him in his kingdom in Heaven.  If Jesus didn’t rise again then our faith is in vain because that means Jesus wasn’t who he claimed to be but we know that isn’t the case because Jesus is the risen Son of God.