Peter Brown writes in his great (and short) The World of Late Antiquity of the years leading up to Christianity’s big break, when Constantine converted in 312:
The Christian Church differed from the other oriental cults, which it resembled in so many other ways, through its intolerance of the outside world. The cults were exclusive, and, often, the jealously guarded preserve of foreigners; but they never set themselves up against the traditional religious observances of the society round them. They never enjoyed the publicity of intermittent persecution. While the oriental cults provided special means to salvation in the next world, they took the position of their devotees in this world for granted. The Christian Church offered a way of living in this world. The skillful elaboration of the ecclesiastical hierarchy, the sense of belonging to a distinctive group with carefully prescribed habits and increasing resources heightened the impression that the Christian Church made on the uncertain generations of the third century. Seldom has a small minority played so successfully on the anxieties of society as did the Christians. They remained a small group: but they succeeded in becoming a big problem.
For men whose confusions came partly from no longer feeling embedded in their home environment, the Christian Church offered a drastic experiment in social living, reinforced by the excitement and occasional perils of a break with one’s past and one’s neighbors.
The suggestion, with a nod to Hans Blumenberg, is that Christianity grew in part through a cultish, gnostic mentality that allowed it to offer a niche, elite appeal. This mentality then had to be discarded by Augustine and the Church itself once the movement had reached a critical mass of acceptance. Yet by Brown’s account, the preparations for such a prioritization of this world were there from the beginning, which is what enabled it to make the transition successfully.