Looking at the passage in
Tacitus, I think there can be no doubt that it is original not interpolated, and that the basic facts related by
Tacitus must be correct:
Nero blamed the fire of 64 on the Christians and executed them in such cruel ways (attaching animal skins to some so they would be torn apart by dogs, burning others alive on crosses at night) as to arouse sympathy for the victims and horror at Nero's own ferocity.
You can't profess to be a serious historian and then make up alleged public events that took place only sixty years before you wrote. If Nero's punishment of the Christians
had never taken place as
Tacitus relates, contemporaries of
his with access to numerous other histories of Nero's time and also with their own or their parents' reminiscences of the true events, would have immediately refuted it and made
Tacitus the laughingstock of intellectual
Rome late in Trajan's reign!
Surprisingly, there have been scholars who dismissed Tacitus' account as an interpolation, or thought for example that it was really the Jews not Christians that
Nero blamed and punished for the fire, but these ideas are solidly refuted by Furneaux in
his 1896
Oxford edition and commentary on
Tacitus, Appendix II, "On the Neronian Persecution of the Christians". I would be interested to hear from
Sejanus on what grounds Michael Grant rejected the factuality of Tacitus' account.
Robert B. says Tacitus' "Christians" may just have been messianists of a different stripe, not followers of the
Christian Jesus, but
Tacitus explicitly says that their name arose from
Christ who was executed by the procurator
Pontius Pilate during the reign of
Tiberius!
As to the "huge multitude" of Christians punished according to
Tacitus, Furneaux comments that this difficulty "may be lessened by remembering that the expression is rhetorical, and that the somewhat similar 'immensa strages' ['immense slaughter'] of 6. 19, 3 has been thought to mean no more than twenty executions in one day."