Tacitus is our only source for the persecution of Christians in 64, and therefore out only source for the earliest state persecution of Christians. Of course, he may have inserted into his narrative an ideological position that was consonant with the reigns of Trajan and Hadrian, under whom he was writing. Or, someone else altogether may have inserted into the narrative an ideological position that was consonant with the late antique times in which he was copying the text. Yet, this unresolvable textual impasse is actually a useful heuristic device, for the effectiveness of sentence Annals 15.44.3 lies squarely in the malleability of conspiracy theory that allows for the easy replacement of historical specificity with utilitarian generality. Conspiracy theory operates by replacement and substitution that can lift Christians from one historical context and drop them into another. Such historical relocation imparts a sense of alarming timelessness and also ubiquity, seen for example in the coordinating conjunction ‘not only across Judaea, the origin of the malignancy, but also across the City, where everything frightful or shameful, of whatever provenance, converges and is celebrated’ (Annals 15.44.3). Through evocations of the Christians, Tacitus can ground the morality of the case against Nero in a complex set of temporal reverberations: What happened in the past (the fire of 64) carries into his present (Rome in the time of Trajan), signalling a constellation of conspiratorial themes that link continually to other times and other places across the empire.
The substitution of general for specific contexts renders conspiracy a-temporal, omnipresent and all the more worrisome. Each changing context (Neronian, Trajanic, late antique), however, involves its own historical specificity and erodes any original sense of the conspiracy, including the questionable culpability of the Christians for the fire. The reference to the Christians in Tacitus is valuable for its potential to bury the traces of a particular situation (the fire of 64) under a self-evident and unquestionable verity (Christians were eventually persecuted). The result is an effortless slip into a timeless and believable, if unproven or unprovable, indictment of Christians as conspirators.
– ‘Conspiracy theories in the Roman Empire’, in Paranoia within Reason: A Casebook on Conspiracy as Explanation (1999), p. 537