From the opening of On the Historicity of Jesus, which is fascinating as a window into the sausage-making of history, apart from the religious questions.
On p. 14 of my copy:
It will not be the aim of this book to debunk apologetic reconstructions of the historical Jesus or the origins of Christianity. I take only secular scholarship seriously – which doesn't mean secular scholars (since a great deal of secular scholarship is produced by the devout), but rather scholars who rely on secular methods and principles of scholarship (a good example being the late Raymond Brown).
Because apologetics differ from scholarship. Apologists ignore methodological distinctions between the possible and the probable in order to maintain the defensibility of a religious dogma. But that isn't how objective scholars behave. If it is realistically possible that Jesus didn't exist, then it is no longer possible to argue that we know he existed. We can only argue that he may have existed, or probably did. This would not be an unusual result in the field of history. But on this specific subject it presents a threat to traditional religion, a threat recognized by Christian apologists, who will disregard facts and logic in opposing it if they have to.