What caused the change in the disciples after the crucifixion?
The resurrection of Christ caused the change. Before it, the eleven disciples scattered in fear at the crucifixion; after Christ rose, they became bold. Peter no longer denied Him, and all were willing to face martyrdom without fear because of the truth of the resurrection.
Les explains that one thing changed the total attitude of the eleven disciples who at the crucifixion scattered and ran for their lives out of fear: the resurrection. After Christ rose from the dead, Peter never again would have kowtowed to a young maid and sworn he never knew Him; Peter stood up to Rome. As near as Les can tell, all eleven apostles were martyred without fear, because of the truth of the resurrection. The same applies to us: we need not worry about what men can do to this body, since they cannot touch the invisible, immortal part that will one day be resurrected. This is what makes all the difference for a Christian—we have the hope of the power of resurrection, first in salvation out of deadness, as Ephesians 2:1 says, 'You hath He quickened, made alive, who were dead in trespasses and sins.' That quickening came by the power of resurrection.