Traditional Christianity claims that a traveling companion of Paul, Luke the physician, wrote the Gospel of Luke and the Book of Acts, and, that the Gospel of Mark was written by John Mark, the traveling companion of Peter, writing down Peter’s memoirs as told to him by the chief apostle. The evidence contradicts this view. See below.
Excerpt from article by Matthew Ferguson, doctoral candidate in Classics:
The way that the Gospel of Luke uses Mark as a source likewise casts doubt on the tradition that John Mark, the attendant of Peter, was the original author of the text. As discussed above, the author of Luke borrows from as much as 65% of the verses in Mark. This is all very interesting, since the author of Luke is likewise the author of Acts, and John Mark, the attendant of Peter, has an appearance in Acts (12:12). This means that the author of Luke-Acts includes within his later narrative the alleged author of an earlier gospel, from which he has even borrowed a substantial amount of his material. Yet, never once does the author of Luke-Acts identify this man as one of his major sources! As Randel Helms points out in Who Wrote the Gospels? (p. 2):
So the author of Luke-Acts not only knew about a John Mark of Jerusalem, the personal associate of Peter and Paul, but also possessed a copy of what we call the Gospel of Mark, copying some three hundred of its verses into the Gospel of Luke, and never once thought to link the two—John Mark and the Gospel of Mark—together! The reason is simple: the connecting of the anonymous Gospel of Mark with John Mark of Jerusalem is a second-century guess, one that had not been made in Luke’s time.
Apologists here will merely try to dismiss this point as being an argument from silence. But again, as in the case of Matthew, the way that the author of Luke uses Mark strongly suggests that he was not “relying” on the recollections of Peter via his attendant, as Blomberg suggests, but was redacting an earlier anonymous narrative. For example, Bart Ehrman in Jesus Interrupted (pp. 64-70) discusses how the author of Luke makes changes to many of the details of the passion scene in Mark. In the Markan Passion, Jesus is depicted in despair and agony, whereas in the Lukan Passion, key details are changed to instead depict Jesus as calm and tranquil during his crucifixion. For example, Jesus’ last words are altered from a despairing statement in Mark 15:33-37 to a more tranquil one in Luke 23:44-46. But why would Luke—the mere Gentile attendant of Paul—redact and change the recollections of Peter—the chief disciple of Jesus—about the passion, crucifixion, and death of Jesus? The reason why is that the author of Luke most likely did not believe that Mark was based on the teachings of Peter. Instead, the anonymous author of Luke redacted and changed Mark, which was written by another anonymous author, to suit his own theological and narrative purposes.
End of post.