The likenesses and unlikenesses of the Synoptic Gospels refuse to fit into a tidy scheme of relationships. Yet, the agreements clearly indicate that some relationship exists.
Today, the majority of scholars hold that Mark was written before Matthew and Luke, that both Matthew and Luke used Mark as a source, and that both Matthew and Luke used a no longer extant document (called Q) as another source. According to Wenham, scholars who critically study the synoptic problem are not particularly impressed by the Two-Source Hypothesis, as it is called, but they find no other hypothesis convincing. A number of scholars have proposed new hypotheses but no new consensus has emerged.
In the 19th century, scholars gave more consideration to a common oral tradition underlying the Synoptic Gospels. Wenham states that 20th century scholars drew the conclusion that if two Gospels share a dozen or more words then there must be a literary relationship between the two writings. But he goes on to note that even modern, Western Christians often memorize passages word-for-word. If people from a culture where information is usually transmitted in writing can memorize passages, then certainly people from a culture where information is usually transmitted orally can do the same. This means the similar wording of individual stories could be explained almost entirely by oral tradition.
But Wenham does not believe the Synoptic Gospels are completely independent of each other. The largely shared order of the material, especially concerning the Galilean ministry where sayings and events are loosely connected both thematically and chronologically, are best explained by recognizing a literary connection. The first Christians were very mobile and it is unlikely that the author of one gospel would not know of the existence of another gospel. The traditions of the early church suggest the Synoptic Gospels were written at different times and that their authors knew of each other’s works.
Wenham finds hypotheses that rely solely on literary relationships or solely on oral relationships to be inadequate. His hypothesis suggests that there is an amount of independence and an amount of interdependence.