In the year 168 AD, the pagan anti-Christian philosopher Celsus quipped that "If all men wanted to be Christians, the Christians would no longer want them." Within 150 years, Christianity was on its way to becoming the dominant religion in the great cities of the Mediterranean.
What accounts for the dramatic expansion of the Christians during the Third Century? Peter Brown posits that the most important factor was a fundamental rethinking by Christian leaders such as of Christianity's relationship to the Roman state and Roman society. They "found that they could identify themselves with the culture, outlook and needs of the average well-to-do civilian." Leaders such as Origen of Alexandria (c. 185 - c. 254) and Eusebius, bishop of Caesaria, thus transformed Christianity from "a sect ranged against or to one side of Roman civilization" to "a church prepared to absorb a whole society."
This is probably the most important aggiornamento in the history of the Church; it was certainly the most decisive single event in the culture of the third century. For the conversion of a Roman emperor to Christianity, Constantine in 312, might not have happened - or, if it had, it would have taken on a totally different meaning - if it had not been preceded, for two generations, by the conversion of Christianity to the culture and ideals of the Roman world.