In Why Christianity Happened (Westminster, 2006), New Testament scholar James G. Crossley adds an intriguing reason for the rise of Christianity to the usual set of factors. It is widely understood that the Roman Empire created a set of conditions conducive to the expansion of the Christian religion, such things as relatively rapid communications and an era of peace and stability. Church historians also point to the person of Christ himself and the quality of church life as factors in that growth. To this list, Crossley adds another factor, which was that an increasing number of citizens of the Empire were attracted to monotheism as a theological choice. Judaism had something to do with the spread of monotheism, but there were also pagan versions as well. Thus, when the Jesus Movement cut loose from Judaism and the cultural restraints it placed on converts, there was a ready pool of people primed in a sense to receive this new religious movement (see pages 99-101).
Like a depressingly large number of "facts" when it comes to the history of the Jesus Movement/church in biblical times, this is a matter of speculation although Crossley makes a good case for the argument that monotheism was an attractive, cutting edge theology at the time. IF he is correct, it suggests that the Jesus Movement took hold because of its congruence with the spiritual temper of its times, not because it stood over against that temper. It spoke to the spiritual aspirations of at least one key segment of Roman society in spite of the political opposition it provoked.
It only makes sense that any viable religious faith must speak to the spiritual and theological sensibilities of an important part of society in its time and place. In our time, that segment of society to which Christianity speaks most fully seems to be increasingly the more extreme end of conservatism, which embraces dualistic doctrines that are increasingly at odds with the growing secularity and multi-culturalism of international culture. It seems to speak best to that segment of society that represents where we have been spiritually and theologically rather than where we are going. That segment is quite large, so Christianity is in no danger of disappearing any time soon; but it does seem that it has lost a key trait that made it the exciting faith of choice for many in Roman times. It seems to be losing its ability to go with the spiritual flow of our age. As I say, in ancient times the Jesus Movement spoke to where we were going; today Christianity seems to speak best to where we have been.
This is, of course, a matter of speculation, but it is not unreasonable speculation and is worth a good deal of thought.