Evidence: The Gathering

[Randy Carson]

Ah. Now I see why you asked.

Have you read Ehrman's Did Jesus Exist?

Yes. But you're (deliberately?) missing the point. Ehrman is an atheist. Coming to the conclusion that Jesus was an actual, living person does not in any way make it more likely that the Gospel accounts of him were any more than a myth created by his followers after the fact, and perpetuated by Paul, as Ehrman's work points out in analysis of the (legitimate) epistles of Paul to the early churches.

Reliance on Tacitus' stated policy of sticking only to documentation and not to "hearsay", when discussing his reference to the Christian sects, ignores that he is essentially discussing them in-passing. His earlier comments about the source of the fires is of no note, as it only references the source he got for the reasons behind the fires, and he is careful to document where he got that information. It is an enormous stretch to take an overall policy when regarding history (as the Annals  are not about religious sects, but about the actions of the emperors) and apply it to a passing description of the claims of those being executed for religious mischief. There is no reason he would have documented the details of their particular religious practices, in his side-reference to the execution of the Christians, during a wider discussion of Emperor Nero. Tacitus saw all cults of Rome as "mischievous superstitions" (etc), because that was his job. He documented the various cults of Rome, and notes the Christian sect as just one more such cult. It is only the importance of Christianity, post-Constantine, that makes us take note of that one passage at all. 

I know you were just citing the author, but I did list volume 15.38 (which he references) in my own citation of Tacitus, specifically to show how his reference to sources was so far removed from his reference to the Christians. Whether he got his information from the records of the testimonies of the Christians about to be executed or from Josephus in their "circles" makes little difference in terms of this discussion, except that Josephus would have known more Christians directly. It sheds no light on the question of whether the information is sourced from (now lost, if they ever existed) Roman records about Pilate and the alleged trial of Jesus. 

What does shed light on the question of whether official Roman records about Pilate were used is the question of why Tacitus (who, as you say, was so careful when dealing with official records) would use the incorrect title of Pilate at the trials. Pilate held both jobs at different points in his career, so it would be understandable if second- and third-generation Christians got his title wrong (Annals was written 80+ years after Christ's alleged execution, and Josephus' Antiquities was 60 years later), but there is no way that the Roman offical account of the trial would get the title at the time of the trial  wrong, when referring to Pilate. Roman accounts recorded at the time of the trial  would not have made a mistake about his present rank, even if it had changed during the course of his career, as some have argued. Arguments that there was no effective difference between Procurator and Prefect ignore that the Romans would not have seen it so, since Tacitus himself records the moment when (in 44 C.E.) Procurators were given the power to govern provinces. This strongly implies that his source material was not a Roman record of the trial events alleged, but of allegations made by later Christians or those who had interviewed Christians about why they believed what they believed. And that is what is seen in both Tacitus and Josephus' accounts, generally (once you remove the obvious interpolations added to Josephus).

TL;dr version - Nothing Tacitus wrote indicates what O'Neill speculates about in his writing, regarding the source of the side-note about executed Christians and/or the historicity of the source of their religious beliefs, in that passage. It does lend credence to the idea that Jesus was a living person, and that his second- and third-generation worshipers at least believed that he was executed by Pilate, but nothing else.

Bart Ehrman discusses Tacitus at length in this post in which he destroys mythicist Richard Carrier:

http://ehrmanblog.org/fuller-reply-to-richard-carrier/

He also discusses the "dying and rising gods" theory.

Let me know what you think.