Although we may never know for sure, the majority of biblical scholars think that Mark was the first Gospel written, and that the other Gospels, especially Matthew and Luke, used Mark as a source. Craig Evans, in The Gospels and Acts (The Holman Apologetics Commentary on the Bible), why this view is the dominant one.
Markan priority appears to be the most prudent position for several reasons: (1) Mark’s literary style sometimes lacks the sophistication and polish often seen in Matthew and Luke. This phenomenon is more easily explained in terms of Matthean and Lukan improvement upon Mark, rather than Markan degradation of Matthean and Lukan style.
(2) In the Markan Gospel Jesus and the disciples are sometimes portrayed in a manner that appears undignified. More often than not these potentially embarrassing passages are touched up or omitted altogether by Matthew and Luke. Again, it is easier to explain the phenomena in terms of Matthean and Lukan improvements upon Mark, rather than the reverse.
(3) The phenomena of agreements and disagreements among the Synoptic Gospels are more easily explained in reference to Markan priority. Among other things, we observe that where there is no Mark to follow (e.g., no infancy narrative, no ‘Q’ material) this is where Matthew and Luke diverge from one another. This observation is more easily explained in terms of Markan priority and Matthew’s and Luke’s independence from one another than in terms of Mark writing last and making use of Matthew and Luke. Markan priority also avoids the problem of trying to explain Luke’s inconsistent use of Matthew.
(4) The small amount of material that is unique to the Gospel of Mark also supports Markan priority. This material consists of 1: 1; 2: 27; 3: 20– 21; 4: 26– 29; 7: 2– 4, 32– 37; 8: 22– 26; 9: 29, 48– 49; 13: 33– 37; 14: 51– 52. In reviewing this material we should ask which explanation seems most probable, that Mark added it or that Matthew and Luke found it in Mark and chose to omit it. The nature of the material supports the latter alternative, for it seems more likely that Matthew and Luke chose to omit the flight of the naked youth (14: 51– 52); the odd saying about being ‘salted with fire’ (9: 48– 49); the strange miracle where Jesus effects healing in two stages (8: 22– 26); the even stranger miracle where Jesus puts his fingers in a man’s ears, spits, and touches his tongue (7: 32– 37); and the episode where Jesus is regarded as mad and his family attempts to restrain him (3: 20– 22). If we accept the Griesbach-Farmer Hypothesis [that Matthew was written first], we would then have to explain why Mark would choose to add these odd, potentially embarrassing materials, only to omit the Sermon on the Mount/ Plain, the Lord’s Prayer, and numerous other teachings and parables found in the larger Gospels.
(5) The final consideration that adds weight to the probability of Markan priority has to do with the results of the respective hypotheses. The true test of any hypothesis is its effectiveness. In biblical studies a theory should aid the exegetical task. The theory of Markan priority has provided just this kind of aid. Not only has Synoptic interpretation been materially advanced because of the conclusion, and now widespread assumption, of Markan priority, but the development of critical methods oriented to Gospel research, such as Form and Redaction Criticism, which have enjoyed success, has also presupposed Markan priority.
In countless studies, whether dealing with this or that pericope, or treating one of the Synoptic Gospels in its entirety, it has been recognized over and over again that Matthew and Luke make the greatest sense as interpretations of Mark; but Mark makes little sense as a conflation and interpretation of Matthew and Luke. The evidence is compelling that Mark represents the oldest surviving account of Jesus’ life, ministry, death, and resurrection. What sources the evangelist Mark made use of, if any, will in all probability remain a mystery. That he made use of some written material seems likely. That he made use of some eyewitness testimony is also probable; it cannot be ruled out.