A substantial majority of modern scholars have come to the conviction...that Mark was written first. Overwhelming internal evidence trumps the tiny bit of late external evidence that exists. Mark is by the shortest of the four Gospels, and more than 90 percent of its information reappears in reasonably similar form in Matthew and/or Luke. At the same time, frequently the episodes common to Mark and at least one of the other Synoptics are narrated with the fullest amount of detail in Mark. If Mark were not written first, we would have to imagine him adding next to nothing to the already existing Gospels of Matthew and/or Luke, abbreviating his overall narrative, leaving out some of the most cherished teachings of Christ from Matthew or Luke (the Sermon on the Mount, the parables of the good Samaritan and prodigal son, etc.), and yet expanding the amount of detail he provided for the passages he did include. And he would have had to do so, often within one and the same passage, by alternating back and forth in terms of when he was following Matthew and when he was following Luke. There were epitomizers of lengthy histories in the ancient Greco-Roman world but none that we are aware of who wrote in such an idiosyncratic way...

These observations have led many Bible students, beginning at least with Friedrich Schleiermacher in the early nineteenth century in Germany, to propose that both Matthew and Luke used a common source for what they share that does not appear in Mark. Because the German word for "source" is Quelle, this hypothetical source has conveniently called Q. That no one has found it preserved independently of Matthew and Luke is not surprising because if its contents were largely or wholly reproduced in these two canonical Gospels, early Christians would have felt no need for preserving it separately.