Here's how I see the logical development:
(1) The Four gospels are not 'independent' in the sense of four strangers eyewitnessing a car accident. This was an old apologetic paradigm, based on the idea of a 'court trial' to establish the facts. But the facts actually show that the four witnesses have previously consulted one another, and colluded with one another. Two things prevent us from calling them 'independent' in the court-case sense.
(a) They are all on the same side, i.e., Christian witnesses, and are obviously promoting their general position. They are not acting as neutral observers in a detached scientific sense, but are actively and enthusiastically promoting Christianity. Their 'eyewitness' testimony is indeed filtered through a special lens not in possession of a mere scientist. This does not negate their honesty or accuracy, but it does change their purposes and interests.(2) It makes a huge difference who wrote first. This becomes obvious from the actual differences between the four gospels. In terms of intent and purpose, and even historical value, it is very important to get it right.
(b) They have plainly plagiarised each other. The Gospels are literary works, and in places have copied one another (or previous documents) word for word. This was previously 'explained' away by the 'dictation' of the Holy Spirit, but the paradigm cannot contain the phenomena.
i.) If the Holy Spirit was the direct cause of identical wording in some places, why would there be such drastic differences in others? It makes no sense as an explanation for the parallel passages, because it doesn't explain the whole picture. This is not a denial of 'inspiration', just a recognition that it is not an explanation for the partial, piece-meal agreements.
ii) At least one author, Luke admits using the work of previous gospel writers. This is a perfectly reasonable explanation for the parallel passages for all cases between gospels. Even if Matthew or Mark don't make the same confession, it can be reasonably assumed, since all the gospels are the same kind of literature.
(a) If we suppose Matthew wrote first, as the Roman Catholics claim, we completely undermine the authority of both Mark and Luke.
(i) If Luke is a 'correction' of Matthew, then the whole 'social gospel' was actually missed or ignored by Matthew, making him a complete jerk and wholly unreliable as an eyewitness, or else Luke has invented this whole aspect, including major parables of Jesus, making either Matthew's or Luke's version historically worthless.
(ii) If Mark were a later 'abbreviation'/synopsis of Matthew: Why would he delete virtually all of Jesus' teaching? He would have to be actively hostile to it, or else wanting to keep it secret. What function could his synopsis serve? Virtually everything in Mark is found also in Matthew (and most in Luke). Mark adds nothing of real significance, and has nothing unique to report. He would be totally 'uninspired' in this very practical sense. His work would be redundant and inferior, especially given its plagiaristic content. Mark, if written last, must be demoted to redundant and unoriginal, a mere editor, unworthy of the honorary title 'gospel writer'.
(b) Suppose Luke wrote first: This cannot save the indictment against Mark. It remains an inferior, pointless production.It can now be seen that Christians have a large vested interest in getting the order of composition of the Gospels right. The authority and credibility of all the gospels depend on it.
(c) Suppose Mark wrote first: This redeems Mark as the first pioneer in a series of expanding and more detailed treatments of the Gospel story.
(i) Now Mark's purpose and intent are honorable. He is writing the newspaper account. He is counting on others who have kept records of the many sermons and other details to eventually provide the full story. He however, has to get the story out. The brevity of Mark's version is reasonably accounted for by his urgency and priority as the earliest gospel.
(ii) Many phenomena are now explained. The fact that Matthew uses virtually all of Mark is understandable. He wishes to preserve everything of importance, and likewise Luke. Matthew and Luke show the real value of Mark by copying him so extensively, and honoring his work, using it as a base for their more ambitious projects. It also explains why Mark was continued to be revered by the early church, in spite of its redundancy. Many of the arbitrary and unimportant details in Mark are edited out of Luke and Matthew however, giving more concise versions of events. This is precisely what we would expect of Luke and Matthew as they incorporate Mark into their 'super-gospels'.
From the Christian point of view, the only question remaining is the relative priority of Luke and Matthew: was Matthew 2nd or 3rd?
I agree that Matthew had other purposes in mind besides dumping the Social Gospel, and going easy on the Jews in Herod's kingdom.
But putting Mark's gospel first chronologically does a lot more than save the reputation of Mark.
Mark as Later Gospel:
Again we point out that if Mark had been written after either Luke or Matthew, he would have been possibly a clever publisher, or even a nice guy, providing a 'Readers Digest' version of the Gospel. But he would have added absolutely nothing to the story Luke/Matthew, except unimportant incidental details of some pericopes. This would be doubly condemning.
(1) Could the Holy Spirit really have "inspired" Mark to delete almost all of Christ's teaching?The answer is no. If we follow Roman Catholic tradition on this, Mark is dead in the water, and the whole concept of a New Testament Canon of four inspired gospels is sunk.
(2) Could the Holy Spirit have "inspired" Mark to add worthless details to those stories he chose to preserve?
Mark as Earlier Gospel:
Thankfully, the evidence itself clearly suggests Mark was written first.
(1) Almost everything in Mark is reproduced in Luke and Matthew, and there is almost nothing unique to Mark. This only makes sense if Mark was used by both Matthew and Luke, and not the other way round. If Mark's purpose had been merely to delete Jesus' teaching, he would be an anti-Christ.
(2) Even though Matthew and Luke have drastically different chronologies between each other, they both reproduce Mark's material in Mark's order! This would be nearly impossible to achieve, unless Mark had either written first, or last. But if Mark had written last, what purpose at all would be accomplished by only copying items out of Matthew and Luke that happened to be in the same order? It would give the appearance of 'agreement', but nothing else of value. This would make Mark a very clever forger, another damning charge against Mark. And he could not have achieved this unless he wrote last, or had the cooperation of either Matthew or Luke (another impossible situation). But the obvious explanation is simply that both Luke and Matthew used Mark as a base, and added their own material in different ways, resulting in a chronological disagreement between them.
(3) Mark includes many explanatory asides, to explain Hebrew/Aramaic terms to a wider audience of Gentiles as well as diaspora Greek-speaking Jews. Most of these have been dropped by Luke and Matthew, as unnecessary. This suggests that Luke and Matthew were written at a later time when there were more people around to explain those terms, and more knowledge among Gentile Christians about Jewish jargon. Adding these to give an artificial appearance of greater age would also be dishonest and suggest Mark was a forgery rather than a real gospel.
(4) Mark is wordier, often adding incidental details typical of an eyewitness account, but of little use or value in the long run. Had Mark written just to add these worthless anecdotes to the gospel, he would be idle, frivolous, and again a forger. But had he written first, it is fully expected that editors like Luke and Matthew would trim the fat from these authentic eyewitness accounts. Again Mark must either have been written first or dismissed as a forgery.
(5) Luke openly says he used previous gospels, and Mark is clearly one of them. Luke uses Mark as an outline, and very simply injects three blocks of new material into Mark. This is clinching. Mark was written before Luke. Luke adds
Block #1: the Nativity StoriesLuke also heavily edits the final section of Mark's Gospel, the Passion Narrative. All in all, these are the features we would expect from a later writer incorporating an earlier account into an expanded and more complete version of the story. Further substantiating evidence is found in the way Matthew has also inserted his unique material in blocks, the only efficient method of doing so.
Block #2: the Sermon on the Plain
Block #3: the large collection of additional teachings.
Luke's Block Outline
Luke block 1: Luke 1 - 2 (Nativity etc.)
Mark block 1: Luke 3:1 - 6:19 = Mark ch 1 - 3 (with suppliments)
Luke block 2: Luke 6:20 - 8:3 (Sermon on Plain etc.)
Mark block 2: Luke 8:4 - 9:62 = Mark ch 4 - 9 (but Luke skips 6:45 - 8:25)
Luke block 3: Luke 10:1 - 18:14 (including Johannine material: e.g., Luke 10:21-22 etc.)
Mark block 3: Luke 18:15 - End = Mark ch 10 - 16 (with minor edits, suppliments)