Christianity in the 3rd century was largely the time of the Ante-Nicene Fathers who wrote after the Apostolic Fathers of the 1st and 2nd centuries but before the First Council of Nicaea in 325 (ante-nicene meaning before Nicaea).
As the Roman Empire experienced the Crisis of the Third Century, the emperor Decius enacted measures intended to restore stability and unity, including a requirement that Roman citizens affirm their loyalty through religious ceremonies pertaining to Imperial cult. In 212, universal citizenship had been granted to all freeborn inhabitants of the empire, and with the edict of Decius enforcing religious conformity in 250, Christian citizens faced an intractable conflict: any citizen who refused to participate in the empire-wide supplicatio was subject to the death penalty. Although lasting only a year, the Decian persecution was a severe departure from previous imperial policy that Christians were not to be sought out and prosecuted as inherently disloyal. Even under Decius, orthodox Christians were subject to arrest only for their refusal to participate in Roman civic religion, and were not prohibited from assembling for worship. Gnostics seem not to have been persecuted.
Christianity flourished during the four decades known as the "Little Peace of the Church", beginning with the reign of Gallienus (253–268), who issued the first official edict of tolerance regarding Christianity. The era of coexistence ended when Diocletian launched the final and "Great" Persecution in 303.