In the prologue to Luke’s Gospel, the Evangelist writes, “Inasmuch as many have undertaken to compile a narrative of the things that have been accomplished among us, just as those who from the beginning were eyewitnesses and ministers of the word have delivered them to us, it seemed good to me also, having followed all things closely for some time past, to write an orderly account for you…” (Luke 1:1-3). This verse provides explicit biblical evidence for what is an implicit characteristic of the Gospels: namely, that the Gospels are dependent on earlier sources.
Luke’s Gospel, like the others, displays a clear use of sources. Almost all NT scholars believe that Luke used Mark’s Gospel as one of his sources. The great majority of scholars believe that Luke also used a document called Q (not to be confused with the James Bond character). Q describes the material shared by Matthew and Luke that is not also found in Mark. However, a great deal of the Third Gospel is unique to Luke (that is, it is not derived from Mark or Q). The three major units of material unique to Luke are his infancy narrative, the so-called “journey to Jerusalem” (comprising much of chapters 10-19, including such famous parables of the Prodigal Son and the Good Samaritan), and the Lukan resurrection appearances. Because of special issues involved with the infancy and resurrection materials, when scholars speak of “L” they most often refer to the stories of Jesus’ earthly ministry that are unique to Luke’s Gospel.
There are as many theories about the origin and character of “L” as there are biblical scholars. Like its Matthean counterpart (“M,” also not a 007 character), this source has received considerably less attention than the notorious Q. But there do appear to be enough similarities within the L material (and enough sufficient differences from Luke’s redaction of Mark and Q) to posit a single source. For a couple good works on this issue, see Kim Paffenroth, The Story of Jesus according to L, JSNTSupp 147 (Sheffield: Sheffield Academic Press, 1997) and D. M. Parrott, “The Dishonest Steward (Luke 16.1-8a) and Luke’s Special Parable Collection,” NTS 37 (1991): 499-515.
So what is “L”? As a result of my research, I currently hold a view similar to Mark Goodacre’s: the combination of both non-Lukan and Lukan features in some of the L tradition suggests that Luke has reworked an existing source (likely oral tradition circulating in Palestine) to add his own distinctive touch. And this original “L” source, I contend, contained an early version of the pericope adulterae.