Sources Behind the Gospels.
As noted previously, there is no simple theory based on copying of one gospel from another that adequately accounts for the observable similarities and differences. We now go on to consider the possibility that all three gospels drew on an older source or sources. As an aside, this will include a discussion of the theory of the lost document known as 'Q' and the supposed 'Gospel of Sayings'. The purpose of this is not to attempt to resolve all such questions: but to show what may reasonably be concluded from the available evidence as to the authenticity of the gospel texts.
Were There Other Sources Apart From the Gospels?
Luke's introduction to his gospel, cited previously, plainly shows that even by the time he wrote (63-70 AD) 'many' different accounts existed. This clearly implies more than just the gospels of Mark, Matthew or John. But while other gospels exist which are clearly of later origin (some of which are nevertheless extremely popular with proponents of 'fringe' beliefs), there are no remaining copies of any earlier attempts.
However, whilst no earlier documents survive, many attempts have been made to deduce what their contents might have been, based primarily on comparisons of similar passages in the synoptic gospels.
The best known of these is a hypothesis assuming that passages common to Matthew and Luke, but not Mark, originate from a lost source known as 'Q'. This has gained such popularity that many talk as if the document actually exists; it doesn't. So-called copies of 'Q' are recent reconstructions of its supposed text. This is very important to keep in mind; because many modern day critics cite 'Q' as if it provides proof of the manner in which they claim the gospels were adapted from earlier sources. But since 'Q' was derived using similar assumptions, it proves nothing beyond the possibility that a similar document, or documents, could have existed and been used as a source.
However, many of these theories go far beyond what may be reasonably concluded into the realms of pure conjecture. The best known of these is the so-called 'Gospel of Sayings'. This is a alleged 'original' gospel that has been derived by a process of extrapolation from the 'Q' text, based upon unproven and widely disputed assumptions as to the type of things Jesus would or would not have done and said. (E.g. it is asserted that Jesus did not do any miracles, or teach about resurrection of the dead, so such passages cannot be original, etc..)
One other document that is of particular interest to scholars, however, is the Gospel of Thomas, which purports to contain secret sayings of Jesus. Although of later date than the synoptic gospels, and evidently adulterated, it shows evidence of having had access to a source similar to the supposed pre-synoptic texts. This makes it potentially useful to textual analysts even though its veracity is suspect. However, a number of modern-day critics have again sought to promote Thomas to a higher standing than it warrants, seemingly in an effort to find an alternative to the testimony of the synoptic texts.
Were Older Documents Deliberately Destroyed?
Although we know there were other collections of sayings and stories of Jesus in circulation when the gospels were written, these documents no longer exist. This loss has been seized upon by conspiracy theorists as evidence of a 'purge' of documents conflicting with a later orthodoxy: but this does not really stand up to critical examination.
From a historical standpoint, there is nothing remarkable about the loss of these documents. Very few texts of this period, sacred or secular, have survived to the present day. The only requirement for non-survival was that later generations felt no great need to keep making copies. By contrast, the amazingly high survival rate of the New Testament documents is undoubtedly due to the exceptionally high regard and wide circulation which they received amongst the rapidly-growing Christian church.
Many of the New Testament epistles predate the gospels themselves; and they openly refer to deviant teachings that were already beginning to affect certain parts of the early church: but there is no hint of any dispute as to the essential facts concerning the life, death and resurrection of Jesus. Quite the reverse, in fact; for Paul in 1 Corinthians 15:1-17 (c. 55 AD) actually cites well-established testimony to Jesus' resurrection as his main argument against some who were suggesting that there would be no general resurrection of the dead. Moreover, scholars who have examined this particular passage have observed that Paul here uses a special rabbinic form of speech indicating that he is quoting a verbal tradition carefully handed down from even earlier times.
Those copies of the gospels which have survived bear eloquent testimony to their widespread distribution (e.g. the earliest known fragment of John's gospel, dated c.130AD, was found in Egypt). Moreover, much of what is known about the later apocryphal writings is known precisely because they did stir up controversy. Had these earlier accounts been of any major significance to the early church, it is extremely doubtful that they could have passed so quietly into complete obscurity.
A Simpler Explanation
So what were they, and why didn't they survive? Far from evoking controvesy, Luke's only criticism of these earlier documents is that they do not present an orderly account of events. This is easily understood if we simply consider the circumstances of Jesus' ministry.
Jesus spent his final years travelling all over Israel, teaching in synagogues, homes and the open air. As was typical of teachers of his day, his sayings were structured in such a way as to enable them to be easily memorised. (Despite the relatively high literacy, for Jewish boys were well schooled, writing materials were few and bulky). Teaching as he did in many places, he would have uttered the same, or similar, sayings on a great many occasions, and these would have become very familiar to his disciples. However, it is likely that some of his hearers would have wished to commit some of these to writing at a fairly early stage.
Similarly with the accounts of Jesus' life. The apostles clearly saw their primary callings to be to pass on their eyewitness testimony to Jesus' words and deeds (cf. Acts 1:21-2). Although in the early days their emphasis was more on verbal than written testimony, it is also quite likely that accounts of Jesus' deeds would have been preserved in various localities, and that some people would have gathered collections of these.
In these circumstances it is virtually certain that collections of Jesus' sayings, and accounts of his deeds, would have been circulating within the early church, in both verbal and written form, from the very earliest days (c.f. Acts 2:42).
But Luke puts his finger firmly on their shortcomings and, by inference, the reason for their disappearance: they tended to be ad hoc collections of sayings and reports rather than systematic accounts. They were of value to the early church as an aid to memory: but once the gospels began to circulate they would have lost their usefulness and been discarded.*
* One exception is the famous account of the woman taken in adultery (John 8:2-11). This is absent from the earliest manuscripts of John; and is almost certainly an isolated text of unknown, but early, origin that was eventually preserved by adding it to the gospel.
Did the Gospel Writers Use Such Sources?
As noted before, the close similarity between much of the content of the first three gospels, in spite of the apparent breadth of choice, does suggest that they used a common body of written or oral materials as a framework for their writings.
Since Luke was not himself one of the twelve, he probably would have drawn on both verbal and written testimony. Although some scholars dispute the authorship of 2 Timothy, it contains an interesting reference (2 Tim 4:13) to certain books and parchments Paul had left at Troas; a place Luke is known to have visited with Paul on at least 2 occasions (c.f. Acts 16:11 and 20:6).
According to the early church Fathers, Mark (not one of the twelve, but a member of the early church in Jerusalem (Acts 12:12,25 et. al.) and later Peter's interpreter) based his gospel on the oral teachings of Peter.
It is not known if Matthew used any sources other than his own recollections: though quite possibly he did. But since Peter was appointed by Jesus himself to lead the early church, it is not surprising if oral or written teachings used as a basis by Matthew and Luke show a close similarity to the teachings of Peter, as presented by Mark.
Since earlier verbal and written sources are known to have existed, it seems probable that the synoptic writers could have used these as a framework for their own accounts. The question of how these documents were organised remains a matter for speculation. However, popular theories that claim they differed radically in substance from the gospels run contrary to the available historical evidence; and tell us more about the personal assumptions of their proponents than the real, historical Jesus.