Yesterday, in the comments, I asked John Armstrong, author of Your Church is Too Small, when he thought the church began to manifest sectarianism. I thought he gave a great answer, so I wanted to share it here:
It seems to be right in the New Testament text from what I can see, at least the root of the matter. But by the late second century is was stretching things and by the third and fourth it manifests itself clearly. Once we get to the latter councils and the icon debates and then the Middle Ages and the 16th century Reformation it becomes full-blown on every side, including the Orthodox and Catholic, not just those who left. I see it as a spirit that results in attitudes and responses to people and the whole need to preserve unity.