Have you actually read any of Ehrman's books? I ask because your statement gives the impression that you've read straw-man versions of what others claim he says,but are unfamiliar with the complexities of what he's actually argued.
I've read several of his books written at the layman's level, and I have no recollection of him ever stating that, "Jesus must have existed because the supposed Paul claimed he met his brother" . Perhaps you could cite a source for such a statement?
I do recall him mentioning the peculiarly of Paul going out of his way to argue as to why his readers should accept his account of Jesus's purpose over the contradictory accounts of people who actually knew him in life. And I recall him pointing out that you can read between the lines of his account of his meeting with Jesus's actual compatriots and see that they were probably happy to see the back of him after agreeing that he should concentrate his efforts on converting gentiles to his version of Jesus's message. But I have never seen him state anything like the argument that you've attributed to him.
Well, based on your post you have not read or have forgotten what is found in Ehrman's Did Jesus Exist? -the Historical Argument for Jesus of Nazareth.
Ehrman argues that Jesus existed based on the claim in Galatians 1.18-19 that the so-called Paul stated he met James the Lord's brother.
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