If one person tells you something strange is true then you might well doubt it and let's face it the claim that someone has risen from the dead is a pretty strange claim. However when there are many witnesses you start to think that perhaps there is something in this. When those witnesses then commit their lives to telling others about it you can start to believe that something remarkable has happened even if you were not there to witness it yourself.
I've already mentioned some of the witnesses but here is some more information.
After the empty tomb had been discovered there was a great deal of confusion.
One of the women, Mary, went looking for the body of Jesus in the garden, or at least went to find someone who might know where the body was. She then saw a man who she believed must be the gardener and so she approached him only to discover that it was in fact the risen Jesus. (John 20: 10-18)
The disciples, terrified for their lives, had gathered together in the upper room of a house. There Jesus appeared to them. (John 20: 19-23)
Similar reports came from others who had met Jesus as they were walking home from Jerusalem. (Luke 24:13-35)
Soon there were lots of eyewitnesses who claimed to have seen Jesus Christ risen from the dead.
Then Jesus appeared to a crowd of witnesses which was calculated to be more than 500. (1 Corinthians 15:6)
To prove that he was not just a ghost or an apparition Jesus ate a meal with his disciples (John 21:12-15) and invited one who was struggling to believe to touch his wounds. This one was called Thomas and after this invitation fell to his knees and said, "My Lord and my God" (John 20:24-31).
Jesus knew that as a person he was limited. His physical presence was a barrier to the next part of his mission. He needed to return to his divine state so he could be with us in spiritual form.
Jesus, God, ascends to heaven so he can send the Holy Spirit, God, to be with us.