Answer:
The persecution of Christians in the Roman Empire occurred throughout most of the Roman Empire's history, beginning in the 1st century AD. Originally a polytheist empire in the traditions of Roman paganism and Hellenistic religion, the Christianization of the Roman Empire brought early Christianity into ideological conflict with the imperial cult and the practice of making sacrifices to the deified emperors, which violates Christianity's prohibition on idolatry; Christians were punished for not conforming to officially-sanctioned religious norms. In the 4th century, the state church of the Roman Empire began persecutions of Christians considered to be apostates, heretics, or heterodox in doctrine.
General persecution of Christians in the empire began with the Neronian persecution under the emperor Nero (r. 54–68), and was resumed under the emperors Decius (r. 249–251) and Trebonianus Gallus (r. 251–253) with the Decian persecution and Valerian (r. 253–260) with the Valerianic persecution. After Valerian's epochal capture by the Sasanian Empire's Shapur I (r. 240–270) at the Battle of Edessa during the Roman–Persian Wars, imperial persecution of Christians was halted by Gallienus (r. 253–260).
The augustus Diocletian (r. 283–305) began the Diocletianic persecution, the final general persecution of Christians, which continued to be enforced in parts of the empire until the augustus Galerius (r. 310–313) issued the Edict of Serdica and the augustus Maximinus Daia (r. 310–313) died. After Constantine the Great (r. 306–337) defeated his rival Maxentius (r. 306–312) at the Battle of the Milvian Bridge in October 312, Licinius and his co-emperor Constantine issued the Edict of Milan, recognizing Christianity as a legal religion. Thereafter, Constantine began his own persecutions of Christians.
After Nicene Christianity was established in the early 4th century, Arian Christians were persecuted by the Roman state, particularly from the reign of Theodosius the Great (r. 379–395) onwards. Theodosius persecuted Arian Christians and established Nicene Christianity as the state religion of the empire. Subsequently, the augusti Pulcheria (r. 414–453) and Marcian (r. 450–457) convoked the Council of Chalcedon and established Chalcedonian Christianity, beginning the state persecution of non-Chalcedonian Christians.