I'm not sure that Charlie Brown and Linus really believed the sound system was working, and the disciples also had a hard time believing that Jesus was alive. Luke says, "While they were talking about this, Jesus himself stood among them and said to them 'Peace be with you.' They were startled and terrified, and they thought that they were seeing a ghost."
The disciples were having a hard time believing. The disciples were trying to sort out the meaning of Jesus' death and the reports about Jesus' appearances. Was it a hoax? They were not completely immune to superstition. Perhaps it was some kind of ghost. Suddenly it happened. Jesus himself stood among them. The disciples were startled and frightened. Then Jesus said to them, "Why are you troubled and why do doubts rise in your hearts? See my hands and my feet, that it is I myself..."
The response of the disciples is a sermon in itself. Luke tells us that they "disbelieved for joy..." It was simply too wonderful to be true. He was alive and he was with them--right there. It was to good to be true.
Some of us may have a hard time believing. Will we allow Jesus to take the doubts away?