The New Testament gospels were not written to be historical books. Subsequent scribes did not in general attempt to ensure changes were not introduced..
Starting with Mark's Gospel, Rhoads, Joanna Dewey and Donald Michie say in Mark as Story (third ed, page 5) Mark should be read as story rather than as history. They say (page 1), the composer of this story has used sophisticated storytelling techniques, developed the characters and the conflicts, and built suspense with deliberateness, telling the story to generate certain insights and responses in the audience. And Burton L. Mack says in Who Wrote the New Testament, page 57, that Mark used pronouncement stories to great advantage in the construction of his gospel, partly because they were the appropriate building blocks for the "life" he wanted to write, partly because they turned on conflict, a conflict basic to the plot Mark wanted to develop, and partly because they were the kind of story that Mark's own community had learned to tell about Jesus. These are not the hallmarks of a historical account.
John Dominic Crossan says in The Birth of Christianity, page 109 a fairly massive (but by no means total) consensus of contemporary critical scholarship Mark came first, and both Matthew and Luke copied from him (but independently of each other). The authors (who were anonymous, in spite of later tradition) may have believed that Mark was a historical account, but this did not stop them from adding nativity, passion and resurrection accounts that fundamentally differ from each other. Once again these were intended as theology, not history.
The Cambridge Ancient History: XI The Imperial Peace A.D. 70-192, page 261, says the Gospel of John is not intended to be read as a biography, but is a mystical and theological interpretation of the life and teachings of Christ. The author draws material from Mark and Luke; doubtless also from independent tradition, but neither the extent nor historical value of such tradition would seem great.
Crossan says (ibid, page 21) each of Luke and Acts of the Apostles, and one no more or less than the other, is theology rather than history. Uta Ranke-Heinemann says in Putting Away Childish Things that Acts is a work of propaganda aimed at Gentile Christians and Gentiles who have not yet become Christians. On page 220, she quotes Ernst Haenchen, who says,
The real Paul, the man known by both his disciples and his adversaries, is replaced by a Paul as a later age thought of him. The earliest days of the Church are not described here [in Acts] by someone who had personally experienced most of it.
Now for some evidence of the ahistorical nature of the gospels:
- In Mark's Gospel, the last twenty four hours in the life of Jesus are neatly divided into eight three-hour segments beginning at 6pm (when it was evening) with the Last Supper, and ending when Jesus was buried in the final period from 3 to 6 pm, before the sun went down. The later evangelists copied Mark's account imperfectly, resulting in this pattern being substantially lost.
- The former Australian Anglican Primate, Archbishop Peter Carnley, wrote of the differing stories of the empty tomb:
The presence of discrepancies might be a sign of historicity if we had four clearly independent but slightly different versions of the story, if only for the reason that four witnesses are better than one. But, of course, it is now impossible to argue that what we have in the four gospel accounts of the empty tomb are four contemporaneous but independent accounts of the one event. Modern redactional studies of the traditions account for the discrepancies as literary developments at the hand of later redactors of what was originally one report of the empty tomb...
There is no suggestion that the tomb was discovered by different witnesses on four different occasions, so it is in fact impossible to argue that the discrepancies were introduced by different witnesses of the one event; rather, they can be explained as four different redactions for apologetic and kerygmatic reasons of a single story originating from one source.
The books of the New Testament were altered, sometimes unintentionally, sometimes with intent, over at least the next three centuries. We know with some certainty of some of these changes, suspect others, and will probably never know about other changes. This is evidence that scribes did not use techniques similar to those used in rabbinic Judaism, to ensure that content would be preserved, and that copies could be authenticated.
One of the most famous interpolations is to Mark's Gospel. It is generally accepted that this originally ended at verse 16:8 with the young man telling the women that Jesus was risen and they fled in terror, telling no one. The ending we now see in Mark 16:9-20 is known as the 'Long Ending', but there was also a 'Short Ending' and some other variants.