Around AD 30, in the province of Judaea, the Roman governor Pontius Pilate had a Jewish teacher from Galilee, Jesus of Nazareth, executed by crucifixion, a punishment Rome reserved mostly for slaves, rebels and non-citizens. Less than three centuries later a Roman emperor was patron of the church that worshipped him. How a small and sometimes persecuted movement became the official religion of the empire is one of history's great turnarounds.

Roman religion

Traditional Roman religion was about practice more than belief. Citizens and magistrates offered sacrifices to many gods, among them Jupiter, Juno and Mars, to keep the peace between the gods and the state, the pax deorum. Rome generally tolerated and absorbed foreign gods, as long as worshippers also honored the gods of the state. In the provinces people also honored the emperor, sometimes as a god, especially in the east, and joining in was a sign of loyalty. Jews, whose religion forbade the worship of other gods, were usually exempted as an ancient people keeping ancestral customs.

Why Christians drew suspicion

Christians, many of them converts from paganism, had no such ancestral standing. Christians refused to sacrifice to the Roman gods or to the emperor, which looked to their neighbors like atheism and disloyalty that might anger the gods and bring disaster. Rumors spread that their private meals involved cannibalism or incest, twisted readings of the Eucharist and of believers calling one another brother and sister. The church grew anyway, mainly in cities, through networks of households and its care for the poor and the sick, and through its welcome to women, slaves and the poor.

Persecution, local and empire-wide

For two centuries persecution was mostly local and sporadic. The first known case followed the Great Fire of Rome in AD 64. Tacitus reports that Nero blamed the Christians for the Great Fire and had many put to death in cruel spectacles; Tacitus despised Christians, yet he thought the killings served one man's cruelty rather than the public good. Tradition holds that the apostles Peter and Paul died in Rome under Nero. Later, around AD 112, the emperor Trajan ruled that Christians were not to be hunted down, though those accused and convicted were to be punished, and that ruling set the pattern for about a century.

Empire-wide persecution came later, in times of crisis. In AD 250 the emperor Decius ordered all inhabitants to sacrifice and to obtain certificates proving it. The Great Persecution under Diocletian, beginning in AD 303, was the last and most severe: churches were destroyed, scriptures burned and clergy arrested. Modern estimates put the martyrs across all the persecutions in the thousands, far fewer than later legend claimed, though many more suffered prison or torture.

Constantine and after

In AD 312 Constantine defeated a rival emperor at the Milvian Bridge outside Rome after, he later said, seeing a sign of the cross with a promise of victory. In AD 313 Constantine and his co-emperor Licinius issued the Edict of Milan, granting religious toleration to Christians and to all others. Constantine funded churches, gave the clergy privileges and summoned the Council of Nicaea in AD 325 to settle disputes over doctrine. Toleration of all soon gave way to favor for one. In AD 380 Theodosius I made Nicene Christianity the official religion of the empire, and in the 390s he banned pagan sacrifice. Some Christians then attacked temples, and pagans and dissenting Christians in turn faced pressure and sometimes violence.

Historians debate why Christianity won. Some stress the emperors' favor. Others note that the church was already growing fast before Constantine, perhaps to a tenth of the population by AD 300 on one well known estimate, because it offered community, moral seriousness, hope of salvation and practical care in plague-stricken cities. The sociologist Rodney Stark argued that steady growth through personal networks, not mass conversions, explains most of it.

Worked example: Pliny's letter

Around AD 112 Pliny the Younger, governor of Bithynia in what is now Turkey, wrote to Trajan for advice. People accused of being Christians were being brought before him, some named in an anonymous pamphlet. He had executed those who persisted, released those who cursed Christ and offered incense before the emperor's image, and found nothing worse among the Christians than what he called a depraved and excessive superstition. Trajan's reply, the ruling described above, added that anyone who recanted by worshipping the gods should be pardoned and that anonymous accusations should be ignored. Apply this level's ideas. Rome did not much care what Christians believed in private; it cared that they would not perform the public act of loyalty. Notice the strange logic: the crime was the name Christian itself, yet one sacrifice could erase it. Trajan's rule against anonymous accusations, a concern for fair procedure, sits beside a readiness to execute people for conscience. The letter shows a state that was neither fanatical nor tolerant in our sense, and it raises a question every society faces: when may a state demand more than obedience to its laws?