Many of the New Testament books claim to be written by eyewitnesses.
The Gospel of John claims to be written by the disciple of the Lord. Recent archeological research has confirmed both the existence of the Pool of Bethesda and that it had five porticoes as described in John 5:2. This correct reference to an incidental detail lends credibility to the claim that the Gospel of John was written by John who as an eyewitness knew Jerusalem before it was destroyed in 70 A. D.
Paul signed his epistles with his own hand. He was writing to churches who knew him. These churches were able to authenticate that these epistles had come from his hands (Galatians 6:11). Clement an associate of Paul's wrote to the Corinthian Church in 97 A. D. urging them to heed the epistle that Paul had sent them.
The following facts strongly suggest that both the Gospel of Luke and the Book of Acts were written prior to 65 A.D. This lends credibility to the author's (Luke) claim to be an eyewitness to Paul's missionary journeys. This would date Mark prior to 65 A.D. and the Pauline epistles between 49-63 A.D.
Acts records the beginning history of the church with persecutions and martyrdoms being mentioned repeatedly. Three men; Peter, Paul, and James the brother of Jesus all play leading roles throughout the book. They were all martyred by 67 A.D., but their martyrdoms are not recorded in Acts.
The church in Jerusalem played a central role in the Book of Acts, but the destruction of the city in 70 A.D. was not mentioned. The Jewish historian Josephus cited the siege and destruction of Jerusalem as befalling the Jews because of their unjust killing of James the brother of Jesus.
The Book of Acts ends with Paul in Rome under house arrest in 62 A.D. In 64 A.D., Nero blamed and persecuted the Christians for the fire that burned down the city of Rome. Paul himself was martyred by 65 A.D. in Rome. Again, neither the terrible persecution of the Christians in Rome nor Paul's martyrdom are mentioned.
These books, Luke-Acts, were written while Luke was an eyewitness to many of the events, and had opportunity to research portions that he was not an eyewitness to.