For many centuries the validity of the NT documents were seldom questioned: but by the beginning of the 20th century it was widely claimed that they had been discredited. Two factors contributed to this change.
a) Probably the most significant was the development of higher criticism.
This is a branch of study which assumes many forms: but is founded upon the concept that stories change with retelling over a period of time. It began to be suggested that this was what had happened to the NT documents.
b) The other significant factor was the gradual loss, over the centuries, of many valuable items of evidence.
Citations in surviving authors testify that these existed: but with them gone, the historical case became harder to prove. As interest in archaeology increased, it became increasingly apparent just how little information had survived.
Moreover, many details of the NT accounts were unsupported by, or even apparently in conflict with, the information believed or known at that time. Also, whilst there were copies a-plenty of the NT documents, none of these could be reliably dated to within a century of Christ, the earlier ones having long since crumbled away.
Given this scenario, it was inevitable that the ideas of the higher critics would gain ground. It has always been recognised that Matthew, Mark and Luke shared much common material (Luke openly comments on his familiarity with other accounts of Jesus’ life); so with a potential timescale of well over a hundred years to work with it was easy to conjecture that various elements of the gospels were added at a later date; these elements being dated on the basis of similarities to ideas expressed in others of the handful of surviving non-Christian sources.
However, whilst higher critical theories have grown more complex, building as these things do on the work of their predecessors, archaeology has also progressed, as has the skill of scholars in correlating and analysing the surviving data. Documents of far greater age have been discovered and analysed, and detailed analysis of documents at the textual (rather than the higher critical) level, together with archaeological evidence, now indicate that Mattthew, Mark and Luke should be dated to within about 30 years of the crucifixion. Even John, normally held to have been the latest of the four gospels, is being reassessed.
From archaeology, numerous discoveries have shed new light on the culture and customs of the period. Not one of these new discoveries has ever been found to contradict the NT records: yet time and again, they have been found to confirm or explain details contained in those documents, showing clearly that the writers were indeed men of the time and place that they claimed to be.
This has, of course, generated quite a bit of opposition from the higher critics, as the timescale is now too short for many of the old theories. Not surprisingly, therefore, there are still a good many scholars who are reluctant to accept these newer findings.
Since a gradual change in the gospel stories due to increasing ignorance of the facts can no longer be reasonably conjectured, it is no longer really acceptable to say, as used to be said, ‘Well, some of it may be factual, but the rest is probably an evolved myth.’ The choice is now much sharper: since the accounts are contemporary with the apostles themselves; either they are an honest account or a deliberate fraud.