Nearly everyone agrees that even though these canonical Gospels are highly problematic as sources for the historical Jesus, they nonetheless do contain some historically accurate recollections of what he said, did, and experienced amid all the embellishments and changes. The question is how to ferret out the historically accurate information from the later alterations and inventions.
Scholars have determined that some of our written accounts are independent of one another—that is, they inherited all or some of their stories from independent streams of oral transmission. It is widely thought, for example, that the Gospel of John did not rely on the other three Gospels for its information. The other three, Matthew, Mark, and Luke, are called the Synoptic Gospels because they are so much alike. The word synoptic means “seen together”: these three can be placed in parallel columns on the same page and be seen together, because they tell so many of the same stories, usually in the same sequence and often in the same words. This is almost certainly because the authors were copying each other, or rather—as scholars are almost universally convinced—because two of them, Matthew and Luke, copied the earlier Mark. That is where Matthew and Luke got a lot of their stories. But they share other passages not found in Mark. Most of these other passages are sayings of Jesus. Since the nineteenth century, scholars have mounted formidable arguments that this is because Matthew and Luke had another source available to them that provided them with these non-Markan passages. Since this other source was mainly made up of sayings, these (German) scholars called it the Sayings Source. The word for source in German is Quelle, and so scholars today speak about “Q”—the lost source that provided Matthew and Luke with much of their sayings material.
Matthew has stories not found in any of the other Gospels, and he obviously got them from somewhere, so scholars talk about his M source. So too Luke has unique stories, and the alleged source then is called L. M and L may have each been a single written document; they may have been multiple written documents; they may have been a combination of written and oral sources. But for simplicity’s sake, they are just called M and L.
And so among our Gospels we have not only Matthew, Mark, Luke, and John (and, say, the Gospels of Thomas and Peter); we also can isolate Q, M, and L. These three were probably independent of each other and independent of Mark, and John was independent of all of them.
In other words we have numerous streams of tradition that independently all go back, ultimately, to the life of Jesus. In light of this fact—taken as a fact by almost all critical scholars—we are in a position to evaluate which of the Gospel stories are more likely to be authentic than others. If a story is found in several of these independent traditions, then it is far more likely that this story goes back to the ultimate source of the tradition, the life of Jesus itself. This is called the criterion of independent attestation. On the other hand, if a story—a saying, a deed of Jesus—is found in only one source, it cannot be corroborated independently, and so it is less likely to be authentic.
Let me give a couple of examples. There is a reference to John the Baptist—a fiery apocalyptic preacher—in close association with Jesus in Mark, John, and Q, all independently. Conclusion? Jesus probably associated with John the Baptist, a fiery apocalyptic preacher. Or an obvious one: Jesus is said to have been crucified under Pontius Pilate in both Mark and John, and there are independent aspects of the story reported in M and L. And so that’s probably what happened: he was crucified on order of the Roman governor Pilate. Or take a counterexample. When Jesus was born, we are told in Matthew (this comes from M) that wise men followed a star to come worship him as an infant. Unfortunately, this story is not corroborated by Mark, Q, L, John, or anything else. It might have happened, but it can’t be established as having happened following the criterion of independent attestation.
A second criterion is predicated on the fact that the accounts found in all these independent sources came down to their authors through the oral tradition, in which the stories were changed in the interests of the storytellers—as they were trying to convert others or to instruct those who were converted in the “true” view of things. But if that’s the case, then any stories in the Gospels that do not coincide with what we know the early Christians would have wanted to say about Jesus, or indeed, any stories that seem to run directly counter to the Christians’ self-interests in telling them, can stake a high claim to being historically accurate. The logic should be obvious. Christians would not have made up stories that work against their views or interests. If they told stories like that, it was simply because that’s just the way something actually happened. This methodological principle is sometimes called the criterion of dissimilarity. It states that if a tradition about Jesus is dissimilar to what the early Christians would have wanted to say about him, then it more likely is historically accurate.
Let me illustrate. Jesus is said to have grown up in Nazareth in Mark, M, L, and John; so it is multiply attested. But it also is not a story that a Christian would have been inclined to make up, because it proved to be an embarrassment to later Christians. Nazareth was a small village—a hamlet, really—that no one had ever heard of. Who would invent the idea that the Son of God came from there? It’s hard to see any reason for someone to make it up, so Jesus probably really did come from there. A second example: the idea that Jesus was baptized by John the Baptist proved discomforting for Christians, because John was baptizing people to show that their sins had been forgiven (“for the remission of sins,” as the New Testament puts it). Moreover, everyone knew in the early church that the person doing the baptizing was spiritually superior to the person being baptized. Who would make up a story of the Son of God being baptized because of his sins, or in which someone else was shown to be his superior? If no one would make up the story, why do we have it? Because Jesus really was baptized by John. Or take a counterexample. In Mark, Jesus three times predicts that he has to go to Jerusalem, be rejected, be crucified, and then be raised from the dead. Can you imagine a reason that a Christian storyteller might claim that Jesus said such things in advance of his passion? Of course you can. Later Christians would not have wanted anyone to think Jesus was caught off guard when he ended up being arrested and sent to the cross; they may well have wanted him to predict just what was going to happen to him. These predictions show both that he was raised—as Christians believed—and that he knew he was going to be raised—as they also believed. Since this is precisely the kind of story a Christian would want to make up, we cannot establish that Jesus really made these kinds of predictions. He may have done so, but following this methodological principle of dissimilarity, these predictions cannot be shown to have happened.