One view of the four gospels is that each is a collection of pericopes that have been selected, arranged and adapted to meet the target audience of a given gospel writer. A pericope in this context is an atomic short story or saying about the Lord. It is generally viewed that these were transmitted orally before the Gospels where produced. It is then considered that when a given evangelist wished to produce a Gospel he would select from the pericopes he knew or could find; arrange them into an order that suited his particular purpose and then made minor edits to fit the text for his target audience.
If this view of Gospel construction is accepted then it has certain ramifications regarding how the Gospels should be interpreted. Firstly it encourages one to compare the four Gospels to see if a given pericope has occurred elsewhere; having done this one may attempt to reconstruct the original pericope in un-adapted form. Secondly it suggests that we need to understand the target audience and context of a given gospel as it may reveal a divinely inspired bias that was used in the adaptation of a given pericope.
It is quite common to find that a pericope view of Gospel construction is tied to a view that one of the Gospels was produced first and that it was partially copied by the other Gospel writers. This view comes about because the degree of agreement between the Gospels, especially the synoptic writers, is far too high at the level of content, grammar and vocabulary to be explainable on the basis of a common oral tradition. Whilst there are many opinions as to which came first the commonest view is that Mark's gospel was first and that Matthew and Luke used that as a basis to add other information to.
It should be noted however that detailed analysis of common passages reveals a pattern of commonality that does not support copying any more than it supports entirely independent production. For example the book "How to Read the Bible for All Its Worth" did a study of the feeding of the 5,000 at the level of the Greek and found the following facts:
These numbers make a certain number of things abundantly clear. John, whilst generally believed to have been written much later than the other three Gospels is an independent source that did not copy much if any of its' content. We also have to assume that John chose to do this despite the fact that he would almost certainly have had access to the other three accounts. We must therefore assume that either John had access to a vastly different pericope to the other three writers or that he was writing from within himself rather than rearranging what had gone before.
The next problem, at least for those who combine pericope logic with Markan priority is that the agreement between Luke and Mark is way too low and the agreement between Luke and Matthew is way too high. We know that the core agreement is about 30% of the smaller two accounts. So how did Matthew and Luke manage to achieve an extra 24% of agreement that didn't go through Mark? Based upon this particular passage one would really be forced to assume Matthian priority but then one is slightly lost to imagine where Mark found the other half of his account. The bottom line from these numbers is that of the four gospels 3 of them have over 50% of their material unique to them.
To me these numbers simply do not support a hypothesis that the writers made significant use of each others material. When my nine year old copies out a 200 word passage for handwriting practice if he makes 20 mistakes he gets to redo the page. Yet to believe that one Gospel author was copying from another is to believe that they tolerated deviances of 40/60 % without any concerns at all. Alternatively we are to believe that the author felt able to alter over fifty percent of what they were copying. But if one felt at liberty to add or delete fifty percent of what was there then why copy at all rather than going to the authority that you felt enabled you to make such gross edits?
Personally I believe it is profitable to leave the field of rank speculation and look to scripture itself to see if we are given any explicit information as to how the Gospels were formed. I believe the principle text here is given in the first four verses of Luke which have helpful pedantic thoroughness:
Lu 1:1 ¶ Inasmuch as many have taken in hand to set in order a narrative of those things which have been fulfilled among us, 2 just as those who from the beginning were eyewitnesses and ministers of the word delivered them to us, 3 it seemed good to me also, having had perfect understanding of all things from the very first, to write to you an orderly account, most excellent Theophilus, 4 that you may know the certainty of those things in which you were instructed.
The first thing to note is that by the time Luke was written there were already many collections of stories and sayings of Jesus in circulation and the text appears to imply that they were compilations and thus written. As most people place the writing of Luke around 60AD this would suggest that the notion that the accounts of the Lord's ministry were primarily an oral tradition is significantly wrong. For many compilations of fragmentary accounts to have sprung up and reached Luke's attention would surely take at least a decade and probably longer.
The next thing to note is that Luke clearly distinguishes between the compilations that were produced and the initial delivery of those accounts by eyewitnesses and ministers of the word. It should be noted that the written compendiums were produced 'just as' the original accounts were given by the eyewitnesses and teachers. This would suggest to me that the original source documents were also produced in written form. Again it is hard to be precise but one could easily imagine that it would take a decade for all of those accounts to be written and move into circulation widely enough to prompt people to start compiling them. This would move the writing of some of these accounts back to at least 45AD possibly even 40AD.
In my opinion it should not shock us to consider written portions of the NT going back this far. In fact I believe the opposite concept should shock us. Whilst the early church had a goodly number of slaves and fisherman it also had a sprinkling of tax collectors, teachers, scholars and rich people. Further a large proportion of the early church was Jewish and thus steeped in the tradition of a written, venerated law. Is it not inconceivable that it would take someone thirty years to come up with the idea of writing down some of the facts relating to the savior of the World? Whilst it may be argued that the persecutions would lead to disruptions would not that very instability crystallize in people's minds the need to preserve certain things for the future?
The next thing to note from Luke's account is that he distinguishes himself from the previous compilers. He claims to have 'perfect' knowledge that allows him to produce an orderly account that people can be sure of. One has to wonder what the 'perfect' knowledge is. If it were just a better collection of pericopes that some of the other compilers then why did the later John ignore them? Does it mean that Luke is including Matthew and Mark amongst the compilers that he is superseding? Or is it possible that Luke's more perfect knowledge actually comes from outside the corpus of written materials that were available to him? Luke certainly claims that his collection is authoritative.
On the basis of the preceding I would offer the suggestion that at least three of the Gospels are not compilations of previous material. John is clearly different purely on the basis of the text he produced. Luke is different because he explicitly states he is different. Matthew is an original apostle and thus has been distinguished from the compilers by Luke. This leaves the issue of Mark. If Mark is really just the amanuenses of Peter then his writings are distinguished from compilations by being the work of an eyewitness. If Mark was a disciple of the Lord and the one that fled during the arrest then he is also distinguished from the compositors by being an eyewitness.
Having stated that I believe three and probably four of the gospels to be independently derived I am faced with the question of how to explain the incredible similarities between sections of some of them. The answer is simple although for some unsatisfactory: divine inspiration. If we are to believe that the God of Heaven can order every cell in the human body to make a functioning whole then it should not surprise us that He is capable of influencing four writers to achieve 60% consistency between four accounts of the same thing. In fact it is the level of deviation that is interesting. It shows that God is remarkably generous in the extent to which He allows the mind of the human author to influence the truth that He is dictating.
The majority of this essay has been an exercise is reductio adsurdum. The belief that the gospels are essentially rearrangements of an oral tradition does not support the mathematics of the similarities seen or the clear testimony of scripture nor human logic applied to the early church. I have shown that at least three of the Gospels are provably independent and I personally believe the fourth is also the result of the direct working of the Holy Spirit. These findings should not of course preclude comparison between the accounts. However I believe it is better to ask what the Spirit was showing on each occasion rather than to question why a given compiler may have chosen to cut and paste a particular account into a particular place.