We have reason to believe that the apostles claimed it because that would be the best explanation for Paul’s experience and his account.  In Paul’s first letter to the Corinthian church (AD 55), he wrote, “For I delivered to you as of first importance what  I also received, that Christ died for our sins according to the Scriptures, and that he was buried, and that he was raised on the third day according to the Scriptures, and that he appeared to Cephas, then to the twelve.”

These creedal clauses can be found in Mark and in Acts describing its early development. In Galatians 2, when Paul describes his visit back to Jerusalem and met with the apostles for the first time he said that they added nothing to his gospel. This traces Paul’s knowledge of the events to within five years of the resurrection.