Luke claims (Luke 1: 2) to be relying on the evidence of eyewitnesses; and the author of 2 Pet. (1: 16) implies that he was an eyewitness at the Transfiguration of Jesus. The latter is part of his apparatus of pseudonymity: it is a way of winning the readers' confidence that the writer is faithfully speaking the mind of Peter.
It has often been held that Luke's statement is a ground for trusting the gospel as a historical record: Luke took the trouble to check on facts recorded in the Acts where he was not himself present, and for his gospel made use of oral and written traditions which in the last resort depended on eyewitnesses. However, the original context of each story had been amended by constant oral repetition before it was set down in the gospels, and all the evangelists were creative theologians who allowed their interpretations to control the course of their narratives; thus Luke altered the time and place and order of the temptations to bring out his own understanding of them; he altered his account of the miracle at Jericho (18: 35); and the Passion narratives each have their own distinctive emphases. If there were eyewitnesses at the foot of the cross, they did not agree about the words of the centurion (Mark 15: 39; Luke 23: 47); it is more probable that Mark and Luke are imposing their own interpretations upon what had been handed down. Sometimes it has been suggested that the presence of vivid details in a narrative—‘on the green grass’, Jesus seated on a cushion in the boat, ‘Jesus wept’—are the faithful recollections of eyewitnesses. But it has been shown that in the course of repeated oral transmission of a story it tends to grow extra details precisely like that!
The gospel according to Mark was once thought to embody what Mark had learnt from Peter. Certainly this was being asserted as early as Papias, a 2nd cent. bishop, but it is clear that much of Mark shows signs of its development within the communities of the Church, such as the explanations given about parables, and the instructions by Jesus to keep quiet about some of his miracles.
There is a claim also in the fourth gospel about the truth of the disciple's testimony (John 21: 24; cf. 1 John 1: 1), and this ‘truth’ must be understood as the truth about Jesus interpreted by and for the Johannine community. The factual history (John 1: 14) is interpreted by the theology. The theological motifs of the writers do not, however, override the historical facts to the extent of turning the NT documents into mere fiction or legends. On the contrary, many of the chronological, geographical, and political details in the Acts are corroborated from other sources; but in the last resort Luke–Acts is controlled by the author's ideas about Judaism and the Jews, the Mission to the Gentiles, the doctrine of the Holy Spirit, and the continuing life of the Church. However, while granting these tendencies, Acts is still a serviceable work of history, with some reliable eyewitness testimonies—but even a reliable eyewitness interprets what he sees.