Christian writers of the period did not dispute Celsus’s observations, but instead tried to explain Christian behaviour and give reasons for Christian non-involvement in secular government. His comments that Christians refuse to enlist or fight in the army are corroborated by all writers of the era that mentioned the subject; for particulars see: www.mpf.org.uk/P21C_34-A.pdf,and www.themennonite.org%2Fissues%2F13-7%2Farticles%2FEarly_Christians_for_p....
Thus we have strong evidence from both Christians and their pagan detractor that believers abstained at least from military service on grounds of religious conscience.
After conceding Celsus’s description of Christian refusal to participate in the military and other government affairs, Origen sought to justify Christian practice by stating that believers provide an alternative service to Emperor and Empire, a service more reliable and valuable than what Celsus urged.
Christians, wrote Origen, do not leave their government bereft of support and aid such that “the affairs of the earth would fall into the hands of the wildest and most lawless barbarians; and then there would no longer remain among men any of the glory of your religion or of the true wisdom” (Celsus’s words). Instead, Origen explained, Christians constitute a powerful army of intercession for the preservation of their secular government through prayer, piety, and good works, whereby its peace is preserved and its armies are successful in holding off barbarian deprivations: “the more any one excels in piety, the more effective help does he render to kings, even more than is given by soldiers, who go forth to fight and slay as many of the enemy as they can.” (Against Celsus 8.73)
The purpose and mission of Christianity is different from that of the secular authorities, wrote Origen. In every country there is a national organization and power besides them, i.e. the church. Through the church believers are led in an enterprise more important than worldly governance. Through it Christians uphold domestic peace, defend against external enemies, and—what is more important—fit people for eternity in heaven (Against Celsus 8.73 and 8.75). Indeed, through the church organization Christians render service for the benefit of everyone, a greater benefit than does the state. Good Christians were encouraged—and some even drafted—into church leadership (Against Celsus 8.75).
The concept that Christians hold citizenship in heaven, with the church as its local authority, had also appeared in the late second or early third century in the Letter to Diognetus 5, by a Christian writer who may have been a tutor to the Roman Emperor Marcus Aurelius: “They dwell in their own countries, but simply as sojourners…every land of their birth [is] as a land of strangers…They pass their days on earth, but they are citizens of heaven.”
According to Origen, Christians seek to establish a new constitution, one in accord with the gospel, which will regulate conduct, if ever they attain secular power. Nor was the possibility too remote that they would form the secular government of Romeand put into practice a distinctively Christian regime that would implement Christ’s teachings. Origen recorded around AD 240-249 that Christians were increasing in numbers and strength, despite persecution by emperors and other pagan rulers (Against Celsus 7.26). Likewise, in the late AD 180s Bishop Irenaeus in France prescribed duties and limits on civil rulers, including that they not act unjustly, tyrannically, impiously or unlawfully (Against Heresies 5.24.2); the fact that he prescribed them can be construed to argue that Christians were already holding public offices, or would be in a position to do so in the not-too-distant future. By the same token, it is hard to make sense of the teaching moderation in secular government in Origen’s On First Principles 3.5.6, or the statement by his teacher Clement in the AD 190s that: “by the counsels of holy men states are managed well, and the household also.” (Clement of AlexandriaStromata 2.19)
A useful summary of the Christian attitude towards Christian-state relations and their differing missions in the second and early third centuries can be found in Against Celsus 8.75:
And if those who govern in the Church, and are called rulers of the divine nation—that is, the Church—rule well, they rule in accordance with the divine commands, and never suffer themselves to be led astray by worldly policy. And it is not for the purpose of escaping public duties that Christians decline public offices, but that they may reserve themselves for a diviner and more necessary service in the Churchof God—for the salvation of men.