Sometimes something's right in front of you for a long time before you notice it. I think I may (finally) have come across a reason to doubt Tacitus' account, in book 15, paragraph 44 of his Annals of how Nero blamed the great fire in Rome in AD 64 on the Christians, who were generally disliked, in order to divert suspicion from himself:
"Sed non ope humana, non largitionibus principis aut deum placamentis decedebat infamia, quin iussum incendium crederetur. ergo abolendo rumori Nero subdidit reos et quaesitissimis poenis adfecit, quos per flagitia invisos vulgus Christianos appellabat. auctor nominis eius Christus Tibero imperitante per procuratorem Pontium Pilatum supplicio adfectus erat; repressaque in praesens exitiablilis superstitio rursum erumpebat, non modo per Iudaeam, originem eius mali, sed per urbem etiam, quo cuncta undique atrocia aut pudenda confluunt celebranturque"
People eager to establish that Jesus existed -- too eager, in my humble opinion -- point to that passage, by Tacitus, who not only was not a Christian but disliked Christians, as evidence that he existed. Other people, who actually want to investigate the matter as opposed to declaring it settled, more reasonably characterize the passage as evidence of the existence of Christians, in Rome, during Nero's reign.
That's how I'd always thought of the passage. I wasn't convinced by arguments that the passage is a later Christian interpolation, or that "Christus" is a misprint and should read "Chrestus," some other guy, not Jesus. [PS, 5. August 2015: My bad, "Chrestus appears in Suetonius' biography of Claudius, not in Tacitus.]
And I'm still not convinced by those arguments, I still find it reasonable to believe that the passage above is reasonably close to how Tacitus wrote it, (The oldest manuscripts we now have containing the passage are from the 15th century, so reasonably close is as close as we're going to get unless and until some much older evidence of Tecaitus' text appears.) and I still see no reason to presume that Tacitus was referring to anyone other than Christians.
And Tacitus has a very good reputation, entirely well-deserved, I think, for being a careful and accurate historian.
But we should never assume that it's certain that any assertion made by any historian is accurate, without looking into the matter a bit for ourselves. What had been staring me in the face for a long time concerning this passage in Tacitus, one of the most closely-inspected and thoroughly-discussed texts concerning the question of Jesus' historicity, without my noticing it, are the following reasons to wonder whether Tacitus may have been mistaken:
Tacitus was about 8 years old in AD 63 when the great fire occurred, and most likely he was not in Rome at the time. In all likelihood there is nothing first-hand about his account of the fire, which was written after AD 100, and maybe as late as 125 or later. Also, many scholars have conjectured that, meticulous and scrupulous as he was, he may have been prejudiced against Nero, and eager to make him look worse than he was. This prejudice may have coincided with a desire on the part of the Christians -- a perennial desire on their part -- to cast themselves in the role of victims. Also, the Christians may have wanted to exaggerate the size and early date of their presence in Rome. I'm picturing Tacitus eagerly taking dictation while a Christian witness eagerly exaggerates things: "Tortured and killed all of you he could find in the most cruel ways he could think of, did he?! Tell me more!"
What really makes me stop and think is that after AD 100, perhaps after 125, writing for an audience many of whom lived in the city of Rome, Tacitus describes who Christians were and where they came from and who their first leader, Christ, had been, and how Christ had died -- in short, he seems to have assumed that his readers hadn't heard of Christians. Does it make sense that in 100 or 125 practically no one in Rome knew who Christians were, while back in the year 64 they were so widely known and disliked that they suggested themselves as natural scapegoats for a disaster?
I'm not sure it does make sense. Perhaps not as much sense as the possibility that Tacitus is an early example of someone taken in by a Christian falsification of history.