What made some of Jesus' followers believe that Jesus rose from the dead? My best guess--and as E.P. Sanders says, we really don't know--is that Jesus' followers--distraught, sad, and disillusioned--had visions in what we now call altered states of consciousness, a condition that can be induced by fasting, sleep deprivation, and fatigue. Eventually, it was conceived that Jesus must have physically arisen from his tomb rather than just appearing spiritually, and so the first gospel, Mark, contains an empty-tomb story, though it lacks any postresurrection appearances, at least in the legitimate ending which concludes with 16:8:
8 So they went out and fled from the tomb, for terror and amazement had seized them; and they said nothing to anyone, for they were afraid.
My thought is that the gospel ends this way on purpose as a way to explain why it took so long to proclaim the empty-tomb message. (Scholars are divided about whether this is how Mark intended the gospel to end or if he had another ending--now lost--containing appearances of Jesus.) When Matthew, Luke, and John wrote later, they added resurrection appearances, though they each followed different traditions regarding where Jesus appeared and to whom. Matthew followed Mark with its implied Galilee appearance (Mark 14:28, 16:7) and thus claimed that Jesus predicted (26:31-32) and fulfilled (28:7-10, 16) an appearance in Galilee, with no mention of an appearance in Jerusalem. Luke and John (except for the later-added John 21, which also has a Galilee appearance in v:1 ff) claim that Jesus appeared in Jerusalem (Luke 24/John 20), and Luke's Jesus makes no prediction about appearing in Galilee (9:22,44; 18:31-34) when speaking about his passion. I think that Paul probably had a vision of Jesus also, though I am not sure of the mechanism that caused this (epilepsy, feeling of shame about persecuting Christians, etc.) and whether Paul's belief that Jesus' rising as a "spiritual body" (1 Cor. 15:44) meant that Jesus' physical body was left behind (thus, no empty tomb, if Jesus was even buried in a tomb) or whether he believed that Jesus' physical body was changed into a "spiritual body," whatever that meant to Paul.
Anyone who has studied this issue and has an opinion about what happened to make the disciples believers, whether there was an empty tomb, what explains the empty tomb, how did Mark's gospel end, etc. please give your thoughts. I only ask that no one preach. I already know what the NT claims happened.