In the case of Jesus, we have someone writing about him as a human being, born of a woman, born as a Jew ("under the Law"), descended from David according to the flesh (one mythicist I spoke to recently resorted to calling that "allegory" in order to avoid Paul's obvious meaning), and crucified. And the last point is crucial, since the idea that someone would invent Jesus and proclaim that he was the Davidic anointed one, expected to restore the kingship to David, but was crucified by the Romans and yet you should believe in him anyway boggles the imagination. Could someone have done it nevertheless. Of course - anything is possible. Is it more likely than not that someone did this, rather than the early Christians engaging in post facto theologizing to try to make sense of why the person they believed was the Messiah was crucified? No, definitely not.
All of this has been discussed here before, hence my tendency to get somewhat frustrated when asked to cover the same ground for the umpteenth time. :)
I can appreciate Dr. McGrath's frustration because I still can't figure out how Paul helps the historicists' case.
Suppose that our earliest source for the Sioux chief Sitting Bull did not know when or where Sitting Bull lived, did not know much of anything that Sitting Bull said or did during his life, claimed that what he did know he learned from Sitting Bull's ghost, and claimed to know others who had encountered this ghost. Suppose that he never claimed that anyone he knew had ever met Sitting Bull during his life, but did refer to certain people as Sitting Bull's brothers. Suppose that this source's interest in Sitting Bull is limited to the activities of his ghost and the only importance he attaches to Sitting Bull is the influence that his ghost has upon the living. Would we consider this source particularly good evidence that Sitting Bull was a historical person rather than merely legendary?
I think this pretty well captures the problems that Paul poses for historicists. His letters don't prove that Jesus didn't exist, but most of what Paul has to say about Jesus sounds much more mythical than historical. When he does describe something about Jesus that can be characterized as historical, Paul does not indicate any source for the information that can be characterized as historical.
Historicists often claim that Paul would have learned about the historical stuff about Jesus from Peter and James or from the cult that he persecuted prior to his conversion. However, this is only true if we have already concluded that Jesus was a real historical person. If Peter and James did not know an actual person, Paul would have learned that. If the cult that he persecuted had worshiped a mythical Messiah, Paul would have learned that. We can't use the assumption that Paul knew whether Jesus was historical as evidence that he was historical.
Perhaps the most frequently cited proof that Paul considered Jesus historical is his reference to James as "the brother of the Lord." In my Sitting Bull example, this probably wouldn't carry any weight at all since the Sioux used "brother" to describe many relationships other than biological ones. Given the rest of the information, we might well conclude that "brother" referred to a relationship with Sitting Bull's ghost. I am not aware of such an expansive use among first century Jews, but Paul does use "brother" frequently enough in referring to spiritual relationships that I think the possibility has to be allowed.
What I think I find most puzzling is the argument that the invention of a crucified Messiah is so mind boggling that a real historical Jesus who was really crucified is definitely more likely. Is their anything more mind boggling than the claims of Joseph Smith or L. Ron Hubbard? History provides plenty of examples of people making mind boggling claims which they attribute to supernatural sources. Sometimes these claims are believed by large numbers of people. In each and every case where it has happened, we would have to assess the likelihood of inventing such a story and having it believed as small a priori, but it happens often enough that I don't see how we can assess the probability that it happened with Paul as definitely less than that of any other scenario. If this is really a crucial point in the case against mythicism, I think Dr. McGrath is going to be frustrated for a long time.
Historians may have perfectly valid reasons for thinking that Jesus was a historical person. I just don't see that Paul helps their case.