Jesus did not just appear to His disciples one time in a ghost-like appearance after His resurrection, but He appeared to them repeatedly, giving them plenty of opportunity to know that it was indeed Jesus of Nazareth with whom they ate and spoke. After one of those times, John remarked,

This is now the third time Jesus showed Himself to His disciples after He was raised from the dead (John 21:14).

Therefore, the disciples were credible witnesses. They knew Jesus before His death. They knew Jesus after His resurrection. Jesus is the Son of the living God.