Johannes Quasten, Patrology, vol. III: The Golden Age of Patristic Literature (Westminster: Christian Classics, 1990) 437.
John Chrysostom was a Christian thinker who lived during the fourth century C.E. In his Homilies on the Gospel of St. Matthew, he states the following about the existence of four Gospels:
Was not one Evangelist sufficient to tell all? One indeed was sufficient: but if there be four that write, not at the same times, nor in the same places, neither having met together, and conversed one with another, and then they speak all things as it were out of one mouth, this becomes a very great demonstration of the truth.
But the contrary, it may be said, has come to pass, for in many places they are convicted of discordance. Nay, this very thing is a very great evidence of their truth. For if they had agreed in all things exactly even to time, and place, and to the very words, none of our enemies would have believed, but that they had met together, and had written what they wrote by some human compact; because such entire agreement as this does not come of simplicity. But now even that discordance which seems to exist in little matters delivers them from all suspicion and speaks clearly in the behalf of the character of the writers (Hom. 1, 5-6 LFC).
Chrysostom's argument is found in conservative Christian circles today. When the four Gospels agree, there are conservative Christians who say, "You see! The Gospels are historically accurate. Here you have four different eyewitnesses, and they each record Jesus saying and doing the exact same thing." And when the Gospels disagree, there are conservative Christians who say, "Well, of course they disagree! That shows that there wasn't any collusion in the Gospels' composition. You'd expect four independent eyewitnesses to differ on the details of what they experienced, right?" How can you argue against that kind of "logic"?
That little rant out of the way, this quote of Chrysostom got me thinking about contemporary New Testament scholarship, or at least the portrayal of New Testament scholarship that I got as an undergraduate (e.g., Markan priority, Q, etc.). Many New Testament scholars would agree with Chrysostom that the authors of the Gospels wrote in different times and places. According to my HarperCollins Study Bible, Mark was written in the late 60's, possibly in Rome. Matthew was written from 80-90 C.E. in Antioch, Syria. The other two are considered late (in the first century), and there is less agreement about where they were written.
On the agreement among the Gospels, many New Testament scholars maintain that there was a sense of collusion (if it can be called that) among the Gospel writers, in that they used each other or a common source in their compositions. According to Markan priority, Mark was the earliest Gospel, and Matthew and Luke used it as a source when they were writing their own narratives about Jesus. For adherents of this view, this would explain why there are times when Matthew, Mark, and Luke employ a common plot-line and the exact same vocabulary in their stories about Jesus. But there are also things that Matthew and Luke have in common that are absent from Mark, so many New Testament scholars posit a Q source, from which Matthew and Luke got the material that they have in common, but which is not in Mark.
That would explain the commonalities among the synoptic Gospels, but how would it account for the differences? If Luke were using Mark, why would he place Jesus' rejection of Nazareth earlier in the story than Mark does? If Matthew followed Mark as a source, why would he decide to have two demoniacs or blind men where Mark presents only one? Scholars can attribute many differences to ideology or historical setting: Matthew changes Mark's story so he can appeal to a more Jewish audience, or Matthew uses different currency in his story than Mark because his audience is more affluent. But what ideological import is there to Matthew having two demoniacs where Mark has only one? Or why would Matthew's resurrection story present women going to Jesus' tomb at "the end of the Sabbath" (Saturday sunset?), whereas the women in Mark go after the sun had risen?
I picked on conservative Christians earlier in this post. Now I want to pick on atheists, or at least the ones who believe in Markan priority and Q. When the Gospels differ, they say, "You see! The Bible has contradictions. It can't be true!" But when the Gospels agree, they say, "Well, that doesn't mean the Gospels were written by eyewitnesses who saw the same events. The Gospel authors used one another as sources. That's why they agree!" How can you win against such "logic"?
Needless to say, New Testament scholarship and the views of conservative Christians and atheists are probably more complex that I present in this post. There are prominent New Testament scholars who don't believe in Q, for example, thinking that sayings attributed to Jesus could have circulated without being in a particular source. And I'm not sure if Markan priority still rules the day in New Testament scholarship. I once heard N.T. Wright dispute it somewhat, for he claimed that Mark could be a shortened version of the other Gospels (or something like that). But I think that there's something to say for Markan priority, considering that the synoptic Gospels often have the same order of events and utilize the same vocabulary. But, again, why are there seemingly insignificant differences among the synoptics, if Matthew and Luke both used Mark?