The above plot from a Peter Turchin blog post, Easter, Early Christians, and Cliodynamics, illustrates a sigmoid curve in the rise of Christianity among Roman elites (elites are relevant since we have data from them). If this is a topic you are interested in, Michelle Salzman’s The Making of a Christian Aristocracy: Social and Religious Change in the Western Roman Empire and Peter Brown’s Through the Eye of a Needle: Wealth, the Fall of Rome, and the Making of Christianity in the West, 350-550 AD are excellent reads on how this transition happened.
Moving away from the autocatalytic model, and describing what happened verbally, in a given population only a minority is strongly motivated on particular details of religion or ideology. Most seem comfortable aligning themselves with the “spirit of the times.” This is true even in the early modern period, as England was forced into Protestantism, while much of Austria and Hungary were dragged back to Roman Catholicism (see Divided by Faith: Religious Conflict and the Practice of Toleration in Early Modern Europe). Only in the 17th century do you start to see populations resisting the demands or preferences of their rulers (e.g., the House of Hohenzollern converted to the Reformed faith but their subjects remained Lutheran, while the Saxons remained Lutheran after the Wettins converted to Catholicism).
What does this imply? The pagans who remained pagan in 450 AD could be more sure about the sincerity and conviction of their fellow dissenters from regnant orthodoxy than pagans from 350 AD. The Christians of 400 AD were less sure about the deep sincerity of the beliefs of their peers than Christians in 300 AD were.