Bart Ehrman describes why he left the faith in his book Misquoting Jesus (8–9):
A turning point came in my second semester . . . . we had to write a final term paper on an interpretive crux of our own choosing. I chose a passage in Mark 2, where Jesus is confronted by the Pharisees because his disciples had been walking through a grain field, eating the grain on the Sabbath. Jesus wants to show the Pharisees that “Sabbath was made for humans, not humans for the Sabbath” and so reminds them of what the great King David had done when he and his men were hungry, how they went into the Temple “when Abiathar was the high priest” and at the show bread, which was only for the priests to eat. One of the well-known problems of the passage is that when one looks at the Old Testament passage that Jesus is citing (1 Sam. 21:1–6), it turns out that David did this not when Abiathar was the high priest, but, in fact, when Abiathar’s father Ahimelech was. In other words, this is one of those passages that have been pointed to in order to show that the Bible is not inerrant at all but contains mistakes.
In my paper for Professor Story, I developed a long and complicated argument to the effect that even though Mark indicates this happened “when Abiathar was the high priest,” it doesn’t really mean that Abiathar was the high priest, but that the event took place in the part of the scriptural text that has Abiathar as one of the main characters. My argument was based on the meaning of the Greek words involved and was a bit convoluted. I was pretty sure Professor Story would appreciate the argument, since I knew him as a good Christian scholar who obviously (like me) would never think there could be anything like a genuine error in the Bible. But at the end of my paper he made a simple one-line comment that for some reason went straight through me. He wrote: “Maybe Mark just made a mistake.” I started thinking about it, considering all the work I had put into the paper, realizing that I had had to do some pretty fancy exegetical footwork to get around the problem, and that my solution was in fact a bit of a stretch. I finally concluded, “Hmm . . . maybe Mark did make a mistake.”
Once I made that admission, the floodgates opened. For if there could be one little, picayune mistake in Mark 2, maybe there could be mistakes in other places as well. . . .
I am convinced that Ehrman is mistaken, not Mark. In the passage Ehrman describes, Mark 2:23–28, Mark presents Jesus making a sophisticated interpretive connection by using the name “Abiathar.” That is, neither Mark nor Jesus is in error. Rather, Mark is presenting Jesus using the name Abiathar in the service of a wider, typological connection. I would invite you to consider the questions I ask about this passage in “The Typology of David’s Rise to Power” (13):
Much discussion has been generated by the fact that Mark 2:26 portrays Jesus referring to “the time of Abiathar the high priest,” when it appears that at the time, Ahimelech would have been the high priest. Goppelt simply asserts: “Mark says Abiathar, but that is an error.” But perhaps there are typological forces at work here, too. David did interact with Ahimelech in 1 Samuel 21:1–9, but Abiathar is the priest who escapes from Doeg’s slaughter (22:20). Could the reference to Abiathar be intentional? Could Mark be presenting Jesus as intentionally alluding to Abiathar’s escape from the slaughter of the priests ordered by Saul and carried out by Doeg the Edomite? Could this be a subtle way for Jesus to remind the Pharisees (“Have you never read,” Mark 2:25) that the opposition to David was wicked and murderous? If this is so, the typological connection suggested by the reference to Abiathar in Mark might be that just as Saul and Doeg opposed David and Abiathar’s household, so also the Pharisees are opposing Jesus and his followers.
In the wider context of this paragraph I discuss the flow of the passage in Samuel and the kind of interpretation Mark presents. Thanks to the generosity of The Southern Baptist Journal of Theology, a PDF of the essay is free to you.
 Goppelt, Typos, 85 n. 106.
 Having come to this position, I was pleased to find a similar suggestion in Rikk E. Watts, “Mark,” in Commentary on the New Testament Use of the Old Testament, ed. G. K. Beale and D. A. Carson (Grand Rapids: Baker, 2007), 141: “If the point is to establish an authoritative precedent, then the actions of Abiathar, as Ahimelech’s son, in taking the ephod to David to become his chief priest and subsequent blessing underscore God’s affirmation of Ahimelech’s decision, his presence with David, and his abandonment of David’s opponent Saul. Not only are Jesus’ disciples justified, but also to oppose them (and, of course, Jesus) is to oppose both ‘David’ and ultimately God, who vindicated him and will also vindicate Jesus.”