Whether or not he had this in mind when he began his journey into apostasy, there's a strategy to Ehrman's attack on the historical Jesus. Basically, it goes like this:
i) Because the text of the NT is unreliable, we don't know what Jesus was actually like. We don't know what he really said and did. Misquoting Jesus.
ii) Even if the text of the NT was reliable, the Jesus traditions which were eventually canonized aren't based on firsthand information. Jesus Interrupted.
iii) Even if the Jesus traditions in the Gospels were based on firsthand information, eyewitness memory is unreliable. Jesus Before the Gospels.
Now, each step in the argument can be challenged. I've done some of that myself, as have others.
But Ehrman's argument suffers from another problem. One way Ehrman attempts to discredit the Gospels is to alleged that some of their claims can be shown to be historically erroneous. For instance, in Jesus Interrupted, he dusts off the chestnut about the census of Qurinius. He says that's falsified by extrabiblical historical sources (pp31-33).
However, a glaring problem with his appeal is that Ehrman is resorting to a double standard. He's exempting the extrabiblical sources from the same skepticism he applies to the Gospels. For instance, he appeals to Tacitus, Josephus, and inscriptional evidence regarding Quirinius. Yet he fails to apply the same criterion to them:
i) Do we have a reliable textual tradition for Tacitus and Josephus? In Misquoting Jesus, Ehman hypothesizes:
Suppose that after the original manuscript of a text was produced, two copies were made of it, which we may call A and B. These two copies, of course, will differ from each other in some ways — possibly major and probably minor. Now suppose that A was copied by one other scribe, but B was copied by fifty scribes. Then the original manuscript, along with copies A and B, were lost, so that all that remains in the textual tradition are the fifty-one second-generation copies, one made from A and fifty made from B.
Although he had the NT in mind when he wrote that, the same principle applies to his extrabiblical sources. What if all our MSS of Tacitus or Josephus derive from a mistake-ridden fifth-generation copy?
ii) Even assuming that we have reliable MSS of Tacitus and Josephus, what's the evidence that their statements about Qurinius are based on firsthand information?
iii) Even assuming that their statements (or the inscriptions) about Qurinius are based on firsthand information, Ehrman has published a new book in which he claims eyewitness recollection is untrustworthy.
So this poses a dilemma for Ehrman: if, on the one hand, he treats his extrabiblical sources with the same skepticism he treats the NT, then he can't use extrabiblical sources as a standard of comparison. By that logic, they are just as dubious as the NT. If, on the other hand, he deems his extrabiblical sources to be prima facie trustworthy, then, in consistency, he must grant the same presumption regarding the canonical Gospels. He can only use extrabiblical sources to impugn the historicity of the Gospels on pain of special pleading. So his trilogy becomes an automated machine that shoots himself in the foot the moment he tries to discredit the historicity of the Gospels by appeal to extrabiblical historical sources.