Extracts below are from Bart Ehrman’s post: Mark: The First Gospel in 19th Century Research dated February 8, 2019.
Over the years, three arguments have proved widely convincing for establishing Mark’s priority to Matthew and Luke:
Patterns of Agreement – The reason then that Matthew and Luke rarely agree against Mark in the wording of stories found in all three is that Mark is the source for these stories. Unless Matthew and Luke accidentally happen to make precisely the same changes in their source (which does happen on occasion, but not commonly and not in major ways), they cannot both differ from the source and agree with one another. The fact that they rarely do differ from Mark while agreeing with one another indicates that Mark must have been their source.
The Sequence of Narrative – Matthew and Luke often present the stories of their Gospels in the same sequence (Jesus did this, then he did that, then he said this, and so on). What is odd is that when they do preserve the same sequence, it is almost always with stories that are also found in Mark. The other materials (mainly sayings) that the two Gospels share—that is, those not found in Mark—are in virtually every instance located in different places of their narratives.
But why would that be? The best explanation is that Matthew and Luke each used Mark as one of their sources and also had a different source that they plugged into the narrative framework of Mark at different places. That is to say, not having any indication from Mark’s Gospel where traditions like the Lord’s Prayer or the Beatitudes would have fit into the life of Jesus, each author put them in wherever he saw fit. Almost never, however, did this other material go in at the same place.
[On 2/12/2025, Dr. Ehman posted to his blog An Argument for Q: The Hypothetical Source That Seems to Have Existed where he explains the above “Sequence of Narrative” observation in more detail. The pdf copy of that post is in his directory in DropBox.]
Characteristics of the Changes – Sometimes Mark uses a Greek style of writing that is somewhat awkward or not aesthetically pleasing, sometimes he uses unusual words or phrases, and sometimes he presents difficult ideas. In many instances, however, these problems are not found when Matthew or Luke narrates the same stories. This difference suggests that Mark was the earliest of the three to be written. That is to say, it would be difficult to understand why Mark would introduce awkward grammar or a strange word or a difficult idea into a passage that originally posed no problem, but it is easy to see why Matthew or Luke might have wanted to eliminate such problems. It is more likely, therefore, that Mark was first and that it was later modified by one or both of the other authors.
A final and related point is that Mark is the shortest of the three Synoptics. If the author had used one of the others as his source, why would he have eliminated so many good stories? Did he want to produce a shorter version of the life of Jesus? This may sound plausible, but a close examination of the Gospel texts shows that it can’t be right: in almost every instance that Mark and Matthew tell the same story, Mark’s is longer. Mark doesn’t appear, then, to be the work of a condenser.
