1 CORINTHIANS 15.5-6: and that he appeared to Cephas, then to the twelve. (6) Then he appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep. (ESV)
The Apostle Paul knew the Apostles appointed by Jesus very well. The Book of Acts records meetings and conversations Paul had with the foremost of the Apostles in Jerusalem. Paul attended the Jerusalem conference described in the 15th chapter of Acts, and in his Letter to the Galatians, Paul describes his conversations and agreement with these Apostles. So Paul knew exactly what their testimonies were of Jesus appearing to them after He was raised from the dead.
John the Apostle writes extensively in the Gospel of John describing a number of post-resurrection appearances Jesus made to John and his fellow Apostles. It is clear what John’s testimony is regarding the revelations of Jesus after He was raised.
The Book of Acts documents the opposition Peter and John and the other Apostles encountered as a result of their express testimony that Jesus had been raised from the dead. The religious leadership in Jerusalem, along with Herod, persecuted the Apostles in an attempt to kill the message of Jesus. Herod had James, the brother of John killed, and he imprisoned Peter with plans to kill him, but God rescued him.
The Apostle John was not killed for his testimony, but he was imprisoned late in his life. The testimony of these Apostles that they had seen Jesus after He was raised from the dead was an essential part of their message. Peter preached about this in his first sermon set forth in chapter two of the Book of Acts.
The message about Jesus was not disputed by the people of Jerusalem. And as Paul points out in verse six above, Jesus appeared to more than 500 additional people. The witness list of people who saw Jesus was extensive.
The Apostles didn’t have disputes with the religious leaders over whether or not Jesus had been raised from the dead. Peter said in his second sermon in Jerusalem in Acts 3.15, “and you killed the Author of life, whom God raised from the dead. To this we are witnesses.” (ESV). The opposing forces didn’t dispute this, they just wanted the Apostles to shut up about it.
The Apostles, however, would not keep quiet about it all and they kept on preaching. They had nothing to gain in any material way from communicating the message of Jesus. They experienced extreme persecution, and in some cases they were killed. The behavior of the Apostles was that of people who were certain about which they spoke.
But their testimony was not always met with opposition and rejection. Acts 2.41 says that after Peter’s sermon, there were 3,000 people who believed in Jesus. And thereafter in the Book of Acts many people believed in Jesus through the testimony and preaching of the Apostles.