It so happens that the Synoptic Problem is one of my favorite topics. The three Gospels Matthew, Mark, and Luke are so similar that it is obvious they have a common written text at their origins. Since they seem to share one vision of Jesus and his ministry, passion, and death. Tradition says that Matthew is that common text having been written by the Apostle Matthew shortly after the Resurrection. It even intimates that it may have been written in Hebrew or Aramaic. Luke's Gospel and its sequel, the Acts of the Apostles, were traditionally attributed to the Gentile physician who was a companion of St. Paul. Mark is attributed to John Mark who worked with Sts. Paul and Barnabas (Acts 12 & 15) and later on in Rome with St. Peter (Cf. 1 Pet 5:13). It was related that St. Mark copied the memoirs of St. Peter for posterity. The dates when the Synoptics were composed are unknown, but (except for the Jerusalem School) no one thinks that Luke came first. Acts was written after Luke, and the story in Acts only goes to @62 AD. It is safe to assume that Luke was written before that time and whatever documents were foundational to Luke had to have been written even earlier.

John has a separate vision of the life of Christ not derived from the same sources though there is some evidence that he was aware of the Synoptic Gospels. Virtually everybody now agrees that he wrote his Gospel @95 AD. His Gospel was considered too spiritual in the early Church and was used a lot by the Gnostic heretics. It was not until 180 AD when St. Irenaeus of Lyon defended John's canonicity that it became widely accepted. Good thing too. It is our primary biblical source for the complexities of Trinitarian theology.

Since the 18th Century, most protestant scholars have thought that Mark (being the shortest of the Synoptics) had to have been written first, and that Luke and Matthew working independently, composed their gospels using Mark and a hypothetical collection of sayings by Jesus which has come to be called Q. This is called the Two Source Hypothesis. As time went on, Q changed from a collection of sayings to another Gospel containing narrative, chronology, and thematic structure. Eventually in the late 20th Century, the pundits decided that Mark had to know Q. That was the point where the Two Source Hypothesis lost all credibility for me. There is absolutely no evidence that Q ever existed except in the imaginations of some theorists. There is no documentation of a "lost gospel" or any inkling that any Gospel text was ever received by the very Early Church other than the four we have today. Q has become a deus ex machina: it contains whatever the theorist wants to find in it to make his theory work.

If Mark had to know Q, then Matthew or Luke could have been the source for Mark without any other document being needed. This accords with Tradition. In fact, if you look at the earliest Church Fathers, the most quoted Gospel in the 1st and 2nd centuries was Matthew, followed by Luke, with Mark giving a very poor showing in third place. It sure doesn't seem that Mark had priority. I favor the Two Gospel Hypothesis postulated by the Lutheran Scholar J.J. Greisenbach 200 years ago and championed in our day by William Farmer, David Dungan, and Dom Bernard Orchard. Basically, this hypothesis says that St. Mathew wrote first, St. Luke used his work as his primary source, and that St. Mark tried to harmonize the two texts to show that here was no substantial difference between them. I think that Matthew was probably written in Hebrew for Jews. Luke was an attempt to compose a Gospel for Gentiles based on Matthew. The material that was included had had different emphases suitable for a Gentile audience. When Matthew was actually translated into Greek, people noticed that there were many discrepancies between the two. St. Peter tried to forestall this problem by helping St. Mark make a harmonization of the other two Gospels supplemented by his own recollections.

It is interesting that Mark can be recited in about 2-3 hours with a natural intermission in the middle. The Actor Alan McCowan noticed this when he gave a dramatic one-man recital of this in Europe and America using the King James Version. Fr. Walter Ong had noticed that the literary conventions in Mark were those of a text designed to be orally recited, not one designed to be read. Mark could be memorized and recited to mixed crowds of Lukan and Matthean supporters each of whom would recognize their Gospel in the narrative.

There are other reasons for thinking that Matthew had priority. The only two Gospels that were written by Apostles who had been with Jesus from the beginning were Matthew and John. Luke and Mark were later converts who did not know Jesus personally. They must have used already published sources because they had no direct knowledge themselves. Also, the material in any two eyewitness accounts is always quite different. When accounts are too similar, they are derivative. The only gospel authors who could have had any eyewitness knowledge of Jesus were John and Matthew. That is why I think their Gospels are the originals and Luke and Mark are derivative. Nevertheless, Luke and Mark contain some unique material so that their portraits are not reducible to a mere subset of Matthew's.

There have been attempts to harmonize the four gospels, but there is one which is particularly notable: the Diatesseron of Tatian composed in the late 2nd Century. While his desire to harmonize the fourfold gospel may have been laudable, the Holy Spirit preferred a more complex picture of Jesus that included some dynamic tension and paradox. The fact that we have four slightly different views of Jesus tells us that we are dealing with a real person who is not reducible to a one-dimensional view.

The Church has always been a little embarrassed at the fourfold Gospel, but in the end I think it is good because mere mortal men can never understand God Incarnate. Any paradoxes we find in the portraits of Jesus are due to our inadequacies and finitude. I hope that helps.