Why do scholars discuss the “Q” source? We might find a recap of “Dating the Synoptics” useful.
When we compare and contrast the Synoptic gospels (Matthew, Mark, Luke), we can divide the materials into three parts:
Mark (even found in Matthew and Luke)
Matthew exclusively (M source) or Luke exclusively (L source)
Verses shared between Matthew and Luke sans Mark.
The passages only found in Matthew and Luke create a puzzle for scholars. What is the relationship between the two gospels in reference to the verses? Does Matthew offer us the key to understanding the problem? Or does Luke? Almost all modern biblical students accept the later. The answer lie in Luke’s unusual editing of the received material, whether from the verses shared with Matthew alone or with Mark.
Luke was an elegant writer of Koine Greek. Compared to the pedestrian language of Mark and John, his prose flowed with flare. He was also a competent editor. His construction of the Infancy Narratives, for example, revealed tight parallels between the announcements and births of the Baptist and the Messiah, linked with transitional passages that highlighted his theme of the Spirit. However, as Luke 1:1-4 stated:
1 Since many (Christians) attempted to put together a narrative about the things having happened among us (and having fully convinced us), 2 just as (the things) handed over to us by the eye witnesses from the beginning and those having become ministers of the Word, 3 it seemed good for me, having investigated all things accurately from (their beginning), to write in an orderly (manner) for you, most excellent Theophilus, 4 in order that you might know exactly about the reliability of the things which you have been taught.
The evangelist took the passages from the “eye witnesses” and “ministers of the Word” seriously. He set his redational skills aside and imported much of Mark’s gospel intact and in order into his own work. He also listed the passages shared with Matthew in seemingly random ways (Matthew, on the other hand, edited the common material into a coherent whole). This presented a dilemma for scholars. They knew origins of Mark. But where did this shared material come from?
In the early part of the twentieth century, scholars (led by B.H. Streeter) coalesced around a theory that these common passages came from a separate source they referred to as the “Q” (or “Quelle,” German for “source”). While they consider it a hypothesis, most scholars today accept the theory at some level; I have too.