Jesus rose from the dead.
He appeared to Mary Magdalene, the disciples on the road to Emmaus and the eleven remaining Apostles that first day. He appeared to many in the days before His ascension. (Matthew 28; Mark 16; Luke 24; John 20-21; Acts 1)
The memory of witnesses was a powerful force in the early decades after Pentecost.
Paul wrote about twenty years later that Jesus appeared to five hundred people at one time after His resurrection and most of them were still alive (1 Corinthians 15:1-8).
Quadratus wrote early in the second century that people were still alive who had been healed by Jesus (recorded in Eusebius. Church History IV,3).
Papias who was born about forty years after the resurrection said he preferred the living gospels to the written ones. Papias had a choice between written Gospels and the testimony of witnesses who were still alive. (Recorded in Eusebius. Church History III. 39)
There are many theories about the origins of our four Gospels. Some involved collections of sayings now lost, or proto versions that have left no copies. A simpler version was told by early Christian leaders. Matthew wrote first for a primarily Jewish audience. Luke completed his Gospel and Acts by the end of Paul’s first Roman imprisonment. He would have had time to study the places and meet witnesses in Judea during Paul’s imprisonment in Caesarea. Mark recorded Peter’s telling of events near the end of his life before his death in Rome. John wrote last near the end of his long life. The best summary of this view and its evidence is David Alan Black’s Why Four Gospels (Energion Publications, 2010).
Irenaeus wrote Against Heresies about one hundred and fifty years after the crucifixion. The Apostle John lived a long life and ordained Polycarp to minister in Smyrna. Polycarp lived a long life and sent Irenaeus to southern Gaul. Irenaeus countered the Gnostic claim that there was secret revelation about Jesus. It was all public. John saw the events and told Polycarp and Polycarp told Irenaeus. He also listed the succession of bishops in different churches founded by the Apostles.
The memory of the eyewitnesses in the decades following the life of Jesus is trumping complicated conspiracy claims and intricate source theories.