Markan Priority is the dominant critical theory used to explain the fact that Mark, Matthew and Luke exhibit a certain conformity of content and narrative superstructure – despite all their differences. The theory dominates for good reason – because it really does offer the best of all possible explanations for this conformity among the three gospels called the synoptics.
To my evangelical friends who would suggest that the Holy Spirit was the likely source of this conformity, I want to say we are talking about a very human type of conformity here which has its share of contradiction and confusion. I question the need of invoking the Spirit to secure such a plainly human outcome, when a perfectly natural explanation is available.
In a nutshell, Markan priority goes like this: A comparison of all three synoptic gospels in unison shows that the Markan structure and language is usually the default position for Matthew and Luke when either one disagrees with the other in content or chronology covered by Mark. This does suggest that these later writers very likely had a copy of Mark’s writing before them, and incorporated as much of this original account into their work as seemed warranted to them by the demands of their own unique material. This editorial process included mostly retention and revision of Mark, and some outright rejection (without much re-arrangement of Mark’s order).
According to my English authority, Vincent Taylor (The Gospel According to St. Mark, 1953), Markan priority was first enunciated in this very general way by C. Lachmann in 1835 and was soon elaborated by C.G. Wilke (1838) and C.H. Weisse (1838). It was finally established as a convincing element in the solution to the synoptic problem by the work of Bernard Weiss (1886) and H.J. Holtzmann (1901). The theory began to attract the attention of British and American critical scholars at the end of the first decade of the 20th Century, but was not treated adequately in a major commentary, according to Taylor, until that of Rawlinson in 1925 (Taylor, pp.10-12).
[NOTE 25 April: as early as the 1907 ICC commentary on Matthew’s Gospel, I find author W.C. Allen making a thorough application of Markan priority to his subject – to brilliant effect.]