How did the attitude of Roman leaders toward Christians change as Christianity spread?

In the apostolic era, Nero is said to have unfairly blamed the Christians of Rome for the Great Fire of Rome. This resulted in persecution of Roman Christians, but apparently Christians elsewhere in the Empire were ignored. Edward Gibbon (The Decline and Fall of the Roman Empire) says that from the death of Christ to the Jewish War, he cannot discover any traces of Roman intolerance, other than the transient but cruel persecution which was exercised by Nero against the Christians of Rome, after the Great Fire.

Gibbon concludes about the general attitude of the emperors to the Christians:
  1. That a considerable time elapsed before they considered the new sectaries as deserving of the attention of government.
  2. That in the conviction of any of their subjects who were accused of so very singular a crime, they proceeded with caution and reluctance.
  3. That they were moderate in the use of punishments; and
  4. That the aflicted church enjoyed many intervals of peace and tranquillity.

He says, "The provincial governors declared themselves ready to listen to any accusation that might affect the public safety; but as soon as they were informed that it was a question not of facts but of words, a dispute relating only to the interpretation of the Jewish laws and prophecies, they deemed it unworthy of the majesty of Rome seriously to discuss the obscure differences that might arise among a barbarous and superstitious people."

By the reign of Septimus Severus (193-211 CE), the increasing numbers of proselytes seems at length to have attracted attention. With the aim of restraining the progress of Christianity, Severus published an edict, which was designed to affect only the new converts, but could not be carried into strict execution without exposing to danger and punishment the most zealous of their teachers and missionaries.

After the death of Severus, Christianity enjoyed a calm of thirty eight years. Until this period they had usually held their assemblies in private houses and sequestered places, but were now permitted to erect and consecrate edifices for religious worship, purchase lands for the use of the community and to conduct the elections of their ecclesiastical ministers in public. The eminent persons of the religion were admitted into the palace in the honourable characters of priests and philosophers.

Severus Alexander (222-235 CE) had the statues of Abraham, Orpheus, Apollonius and Jesus placed in his domestic chapel, as an honour justly due to respectable sages.

Decius (249-251 CE) condemned Christianity as a recent and criminal superstition and began the first confirmed, widespread persecution of Christians. The bishops of the larger cities were removed by exile or death and the clergy of Rome were prevented for sixteen months from proceeding to a new election for a bishop of Rome.

The next period of widespread persecution began under Valerian (253-260 CE ) At first, he was noted for his clemency towards the Christian faith, but in the last three and a half years, matched the severity of Decius.

Valerian was succeeded by Gallienus who addressed an edict to the bishops, in such terms as to acknowledge their office and public character. Without formally repealing the ancient laws used against Christians, he allowed them to lapse.

The Great Persecution of Christians took place under Diocletian (284-305 CE). During the first eighteen years of his reign, there was a continued spirit of toleration towards Christians. Many senior officers of the palace, although they might sometimes be required to accompany the emperor when he sacrified in the temple, were nevertheless Christians. During this time, bishops held an honourable rank in their provinces and were treated with distinction and respect, not only by the people, but by magistrates themselves. This was a time when pagans were becoming incensed at the presumption of a recent and obscure sect to accuse them of error and to declare their ancestors to have been condemned to hell. Refusal of some Christians to serve in the army as long as it was led by a pagan emperor, assisted in alienating the emperors.

It seemed to Diocletian and his fellow-emperors that the Christians, renouncing the gods and institutions of Rome, had begun to constitute a distinct republic that needed to be suppressed before it had acquired any military force. It was already governed by its own laws and magistrates, was possessed of its own public treasury and held frequent assemblies of the bishops, whose decrees were implicitly obeyed by their congregations.

It was enacted that the churches, in all the provinces of the empire, should be demolished to their foundations; and the punishment of death was denounced against all who attempted to hold any secret assemblies for the purpose of religious worship. By the same edict, the property of the church was confiscated and either sold to the highest bidder, united to the Imperial domain, bestowed on the cities and corporations, or granted to rapacious courtiers. Judges were authorised to hear and to determine any action that was brought against a Christian, but the Christians not permitted to complain of any injury which they themselves had sufferred.

Within fifteen days of Diocletian's edict being published, his palace at Nicomedia was twice in flames. Both times the fires were extinguished without any material damage, but the repetition of the fire was justly considered as an evident proof that it had not been the effect of chance or negligence.

Diocletian had no sooner published his edicts against the Christians than he abdicated. Within a few years, Christianity was in the ascendancy and the long period of persecution of the pagan temples began.
Constantine the Great supported the Christians. He built Christian churches, promoted Christians in the imperial administration, introduced some laws favourable to the Christians and convened Christian councils to arbitrate disputes between rival Christian doctrines. All but one of the emperors after Constantine were Christians. They introduced anti-pagan laws. The co-emperors Gratian and Theodosius I made mainstream Christianity the state religion of the empire and banned dissident Christian doctrines. Theodosius I introduced harsher anti-pagan laws and persecuted the dissident Christians and the pagans