06-27-2016, 02:58 PM
Like many historical things, it was a development that didn't have just one single starting point. There were a series of watershed moments that can be debated as to which were more or less significant.
If I have to pick a single event, I would pick either the Qunisext Council in the late 7th century or the Second Council of Nicaea in the 8th century. In the former case, its significance is that it's when the Pentarchy, which is a defining feature of Eastern Orthodox as it has come to be defined since then (parallel to the Pope for Roman Catholics), came about. In the latter case, I would say that, since it's the most recent council that Eastern Orthodox consider to be ecumenical, and so agreement with it must be a sine qua non of Eastern Orthodoxy, and of course it could not have been a sine qua non of Christianity before that time since it hadn't happened yet, and (as we can easily demonstrate) many earlier fathers, even among those who are revered as saints in Eastern Orthodoxy, wouldn't have accepted its dogmas.
Another way I might approach it would be to go back to when a more unified organization out of which both Roman Catholicism and Eastern Orthodoxy developed and went their separate ways. Looked at this way I would probably pick the tenure of Pope Gregory the Great around AD 600, since he, more than anyone else, moved the papacy toward the pretense of authority over the universal church that is such a defining feature of Roman Catholicism.