First off, in his book "Did Jesus Exist?", he does Christianity a great service by arguing that Jesus did in fact exist in the first place. While this was never really a question (almost no scholar would take the radical position that Jesus did not exist), there are a lot of easily-led layfolk out there who would jump on the bandwagon of "Jesus-mythers" like Richard Carrier. So it is nice to have someone who writes at the popular level, and who has the readership from the internet atheist audience as Dr. Ehrman does, putting that myth (of Jesus being a myth) to rest.
Second, even in his books that directly attack Christianity, he gives a lot of ground to the historical credibility of Christianity. For example, in his latest book, "How Jesus Became God," Dr. Ehrman admits some historical facts that are very important to Christianity. A few of these are:
- Jesus was a real historical person, a Galilean Jew who preached the kingdom of God. Ehrman has devoted a whole book to defending this fact.
- The canonical Gospels are the earliest and, for all practical purposes, the only valuable sources of detailed information about the historical Jesus. The “Gnostic” gospels and other apocryphal writings date from much later and are not significant sources of historical information about Jesus.
- Jesus thought he was, or at least would become, the Messiah.
- Jesus was crucified at the order of Pontius Pilate.
- Jesus actually died on the cross.
- Some of Jesus’ original followers sincerely believed they saw Jesus alive from the dead.
For more on this see:
That blog post quotes this other, more comprehensive blog post on Ehrman's newest book: