Why were Christians persecuted, and what were they accused of?

Emperor Trajan 53–117 ADLetter 10.97, to Pliny 10.97.1–2

They are not to be sought out. If they are reported and proved guilty, they must be punished, but in such a way that whoever denies he is a Christian and makes it plain by deed, that is, by worshiping our gods, shall obtain pardon for his repentance, however suspect he was in the past. Anonymous accusations must have no place in any charge; they are a very bad precedent and not of our age.

The policy for over a century: no hunting, but death for the confessed name. Tertullian mocked it: if they are guilty, why not seek them; if innocent, why punish them (Apology 2.8)?

TacitusAnnals 15.44.2–3

But neither human help, nor the emperor's generosity, nor appeasing the gods removed the infamy that the fire was believed to have been ordered. So, to scotch the rumor, Nero fastened the guilt on others and inflicted the most exquisite punishments on those whom the crowd called Christians, hated for their shameful acts. Christus, from whom the name comes, was executed by the procurator Pontius Pilate in the reign of Tiberius; and the deadly superstition, checked for the moment, broke out again, not only in Judea, the origin of that evil, but also in the city, where all atrocious and shameful things from everywhere flow together and are celebrated. Therefore those who confessed were first arrested; then, on their information, a huge multitude was convicted, not so much on the charge of arson as for hatred of the human race.

Tacitus describes Nero's scapegoating of the Christians after the great fire of Rome in AD 64. He despises the Christians and calls their faith a deadly superstition, yet he is the key pagan witness both to the execution of Christus under Pontius Pilate in the reign of Tiberius and to the conviction of Christians for hatred of the human race.

Athenagoras fl. c. 177 ADPlea for the Christians 3.1

Three charges are brought against us: atheism, Thyestean feasts, Oedipean intercourse. If these are true, spare no class of us; punish the crimes; destroy us root and branch with our wives and children, if any Christian lives like a beast.

"Thyestean feasts" means cannibalism (a garbled picture of the Eucharist); "Oedipean intercourse" means incest (from "brothers and sisters" and the kiss of peace). "Atheism" meant refusing the gods.

Tertullian c. 155–c. 220 ADApology 40.1–2

They think the Christians are the cause of every public disaster and every popular misfortune. If the Tiber rises to the walls, if the Nile does not rise over the fields, if the sky stands still, if the earth moves, if there is famine, if plague, at once the cry goes up: "Christians to the lion!" So many to one lion?

Neglect of the gods was thought to anger them and endanger the city.

Caecilius (pagan speaker in Minucius Felix)Octavius 9

I hear that they venerate the consecrated head of a most vile beast [an ass], under some foolish persuasion — a religion worthy of such morals, and born from them! Others say they worship the genitals of their own chief priest and adore his nature as that of their parent: whether it is false I do not know, but it is certainly a suspicion that fits hidden, nocturnal rites. And whoever says that a man punished with the utmost penalty for a crime, and the deadly wood of the cross, form their ceremonies, assigns to depraved and criminal men altars that suit them, so that they worship what they deserve. ... An infant, covered with meal to deceive the unwary, is set before the one who is to be initiated into the rites. The infant is killed by the novice — lured by the surface of the meal as if to harmless blows — with blind and hidden wounds. His blood, oh abomination, they greedily lick up; his limbs they eagerly divide among themselves; by this victim they are bound together, and by this shared knowledge of the crime they pledge themselves to mutual silence.

These are not Minucius's own charges: they are the speech of Caecilius, the pagan interlocutor in the dialogue, retailing rumour (“I hear”, “others say”, “whether it is false I do not know”). Octavius answers the speech at length from chapter 16 onward. The passage shows what educated pagans believed Christians did in secret.

Eusebius of Caesarea c. 260–c. 339 ADChurch History 8.2

The houses of prayer thrown down from their height to the ground, together with their very foundations, and the divine and holy Scriptures handed over to fire in the middle of the marketplaces, we saw with our own eyes. ... Imperial letters were spread everywhere, ordering that the churches be razed to the ground and the Scriptures destroyed by fire, and proclaiming that those who held honours should be degraded, and household slaves, if they persisted in their profession of Christianity, should be deprived of their freedom.

Eusebius was living in Palestine when Diocletian's first edict (303) opened the Great Persecution, and he writes as a contemporary eyewitness, not a later compiler. A second order soon followed in the same chapter: every church leader was to be imprisoned, then forced by every means to sacrifice.