The Roman Empire initiated several persecutions against Christians over the centuries. The most infamous occurred under Nero in 64 A.D. However, the most widespread assault began two centuries later. In 303, Diocletian ordered the most far reaching and bloody persecution of Christians in Roman history. In the end, Diocletian's efforts failed, the church grew, became legal in 313, and eventually Christianity became the empire's official religion.
Diocletian elevated to the imperial purple in 284. The new emperor did not care for Christians, but did not openly attack the religion. Rather, he moved against Christians in the government or trouble makers. Over the first two decades of his reign, Diocletian expelled Christians from the military, appointed anti-Christian politicians to positions of power, and ordered the execution of Gnostics. The emperor worked to restore Rome to its former glory and Christians had no role in Rome's greatest moments.
The emperor's imperial ideology involved fervent religious conservatism. Diocletian wanted to honor the old Roman gods and cult of the emperor. He did not create, perpetuate, or advocate anything new. Instead, the emperor wanted to restore the old religion as he wished to restore the old empire. In 302, Diocletian's partner Galerius advocated a renewed persecution. Diocletian remained unconvinced of the need for a purge. He changed his mind after a visit to the Oracle of Apollo. He called for a general persecution in February 303.
To begin with, Diocletian ordered the destruction of a new church in Nicomedia and the impound of any treasure. Then, he issued his first edict of persecution. The emperor targeted church property and clergy. Authorities were ordered to destroy books, churches, and religious symbols. Christian services were banned. On top of this, Christians had no legal recourse and became eligible for torture. Although Diocletian wanted to avoid bloodshed, Galerius ordered rebel Christians burnt alive.
The persecution led to rebellions in parts of the empire. By 300, there might have been 6 million Christians under the emperor's rule. Diocletian responded with a second edict that ordered the imprisonment of all clergy. He modified his position in November 303 by offering a general amnesty to any priest that sacrificed to the gods. This was unacceptable to many Christians. In 304, Diocletian ordered all people to appear in public squares for sacrifice under penalty of death.
The emperor retired in 305. By this point, Diocletian's tetrachy ruled the empire. Eastern emperors continued with the persecutions while western emperors appealed to Christians for support in a series of civil wars. Some may have believed Christianity on its deathbed by the time Constantine emerged. However, the upstart used the Christian god to unite his troops and gain power. In the end, Constantine legalized the outlaw faith and made it Rome's official religion.
The Diocletianic persecution failed completely. There were simply too many Christians in the empire. By this point, many pagans sympathized with the persecuted and admired their bravery in the face of horrific deaths. There is no way to know the number of martyrs in the Great Persecution. It might have been as low as 5,000 or as high as the tens of thousands. In the end, Christianity became the empire's religion and the glue that held the Romans together. As a result, when the empire collapsed, the church infrastructure kept western civilization alive for centuries.