Without doubt, Mark's Gospel is the foundation of the Synoptics. It is the earliest of the three, and both Matthew and Luke broadly follow its chronology and framework. As such, its impact on the Synoptic tradition is hard to overstate. The question is thus thrown into sharp relief: "Does it contain reliable, perhaps even eyewitness, testimony?"
I. External Evidence: Tradition
Tradition holds that Peter is the primary source for Mark's Gospel. The source of this position can be traced to Papias, the Bishop of Hierapolis. Reportedly writing in the early second century, Papias says:
The Elder used to say: Mark, in his capacity as Peter’s interpreter, wrote down accurately as many things as he recalled from memory – though not in an ordered form – of the things either said or done by the Lord. For he neither heard the Lord nor accompanied him, but later, as I said, [he heard and accompanied] Peter, who used to give his teachings in the form of chreiai, but had no intention of providing an ordered arrangement of the logia of the Lord.This statement has ambiguous elements; for example who is the "he" who "recalled from memory?" Was it Mark or Peter? What exactly is meant by chreiai, often explained as a "pithy teaching through historical reference or reminiscence?" What exactly is meant by "logia", often considered "sayings" or "words?" And what on earth does Papias mean by "ordered arrangement?"
Though we cannot be certain, perhaps the best interpretation of this statement is that Peter used to give Jesus' teachings in pithy accounts and anecdotes, but that he never gave a chronological ordering of these accounts. Mark wrote these sayings down accurately.
The date of this statement can be misleading. Papias may have written this statement around 110 AD, though there are some who question the dating and say it was written at the end of the first century. Regardless, it appears that the origin of the statement is even earlier. Note how Papias begins: "The Elder used to say..." In other words, Papias is reminiscing about an earlier time, when the Elder bore witness to Mark. Given the potentially earlier dating of Papias and a time lapse between the statement of the Elder and Papias' account, it is quite possible that the tradition of Peter's eyewitness testimony in Mark's Gospel was established in the mid-late first century, just decades after Jesus' death and relatively concurrent with the writing of the Gospel itself.
II. Internal Evidence: Early Dating
Mark was written quite early after Jesus' crucifixion. Although a date as early as the 50's has been argued for Mark, there is no need to argue such a position to simply posit that Mark knew Peter. The scholarly consensus for the date of Mark is around 70 AD, and the most common dates provided for the date of Peter's death are 64-68 AD. Chronologically speaking, this makes it quite possible that Mark knew Peter and received his testimony.
It is also quite possible that portions of Mark's Gospel are far earlier. Every now and again there are names that are mentioned in Mark which seem rather irrelevant. Take, for example, the name of Simon of Cyrene (15:21). He is quite pertinent to the story, and we expect to see his name. But Mark, and only Mark, includes the name of Simon's sons: Rufus and Alexander. Why include their names? It has been posited that their names are being provided as references by Mark. The reader and listeners of the Gospel are being told whom to contact for confirmation of this portion of the story. According to this theory, Rufus and Alexander were alive and around at the time Mark was written. If this theory is accurate, it indicates a very early date and acquaintance with key persons. This increases the likelihood that the author knew Peter.
Conversely, we see some names being omitted from the account which one would perhaps expect. During Jesus' arrest in the Garden of Gethsemane, "one of those who stood by" assailed the servant of the high priest (14:47). We learn from John that the assailant was none other than Peter (Jn 18:10); why does Mark not mention Peter's name? A similar point can be made for the boy who "escaped naked" from the scene (14:51-52). Why was his name not given?
It has been argued that these actions would have caused the men in question to be wanted by the law; Peter for attacking the servant and the boy for evading arrest. Mark, opting not to incriminate the men by identifying them, chooses instead to leave their names anonymous. In other words, he omits their names for the sake of protective anonymity. If this theory is accurate, then we have yet another reason to believe that sections of Mark's Gospel are written very early. In fact, this theory also provides reason to believe that Mark wrote in Palestine, for why would Mark be concerned about naming these men in an outside jurisdiction? Both of these factors significantly enhance the probability that Mark knew Peter. (See Gerd Theissen, The Gospels in Context, for more details about this argument).
III. Internal Evidence: Literary Devices
Some scholars, such as Richard Bauckham, have argued for the use of inclusio in Mark's Gospel. This is a literary device which indicates who the source of an account is to the audience. By placing a person's name at the beginning and end of an account, the author is identifying the primary source. Bauckham, in his book Jesus and the Eyewitnesses, argues that this device was used by other Graeco-Roman authors including Lucian in his biography Alexander and Porphyry in his work Life of Plotinus. He then argues that Mark used the same device when providing Peter's name immediately after Jesus Baptism (1:16) and by providing his name at the end of the Gospel (16:7). If Bauckham's theory is accurate, then Mark himself indicates that Peter is the primary eyewitness for his Gospel.
Peter's Narratological Voice
When we examine the manner in which Peter speaks throughout the Gospel of Mark, we see that his voice functions in the way a narrator's voice would. For example, Peter often speaks for all the disciples (eg. 10:28; this can only be taken so far, though, since it seems John might be doing this in 9:38). At other times, Peter represents all the disciples (e.g. 14:37). At other points, a response that should only be directed at Peter is directed to the whole group (e.g. 8:30, 11:22). Equally striking is the fact that Peter, though often representing the group, also displays the most individuality throughout Mark's Gospel. The effect is similar to a first-person account as told by a member of a crowd. Given that the narratological voice of Peter fits what we would expect if Peter were the primary witness of Mark, this constitutes internal evidence.
It has also been noted that the perspective of the narration seems to be that of an insider, probably Peter. This can be noted in at least three ways.
First, in a 1925 journal article, C. Turner argues that Mark's narrative is told from the perspective of one of the twelve disciples, probably Peter. The crux of his argument is that throughout Mark, unconventional verbiage indicates that a first person account was conformed to a third person narrative. It seems as if the author has taken first person plural "we" statements, particularly statements describing movement of Jesus and the disciples, and turned them into "they" statements.
An example of such a statement is "They went to a place called Gethsemane, and he said to his disciples..." (14:32) This switch from third person plural directly to Jesus is awkward when "they" is used, but if it is understood that one of the disciples is speaking about their movements with Jesus, then the statement becomes much more natural: "we went to a place called Gethsemane, and he said to us..." One additional example is 1:39. The verse reads "they left the synagogue and came into the house of Simon and Andrew with James and John." If Simon Peter is understood to be the speaker, this verse reads much more naturally: "We left the synagogue and came into our house with James and John." Thus, Turner argues for a Petrine origin through narrative perspective.
A second reason to think that Mark's narration was provided by an insider is the perspective of the "Inner Circle" accounts. In certain cases, such as the Transfiguration and the Garden of Gethsemane, the only witnesses present to vouch for the account are Peter, James, and John. In these two specific cases, Peter takes an active role and stands out from the other two, quite possibly since he is narrating his own perspective.
Perhaps a third reason is that in certain circumstances, Peter is the only viable witness to an event which is recorded; this is the case in his denials of Jesus. Not only is no other person present in the narrative to see his denials, it is quite possible that no one other than Peter would dare tell the account of Peter's denials. Who would wish to risk denigrating Peter, a leader of the church in Jerusalem, so early in the movement? It makes sense that it would be Peter himself, in humble admission, who tells this story despite the embarrassment.
In sum, the perspective of the narration seems to be that of an insider, someone among the disciples, most likely someone from among the inner circle of three. Given the details of the narration, the most likely candidate is Peter.
The above arguments show that external and internal evidence work together to point to Peter as the primary witness for Mark's Gospel. Can we be absolutely certain that Peter is the primary eyewitness to Mark's account? Certainly not without addressing the counterarguments. This article just provides the positive case; to take a stand, one ought to study the opposing case as well. It is my prayer that this article might motivate you into studying the issue with more depth.
For now, it is safe to conclude that there is very good reason to consider Peter as the primary eyewitness of Mark's Gospel.