Nestorius was at one point a leading theologian in Constantinople, he was trained in Greek philosophy, and his heresy, like many others of the period, derived from attempts to apply these philosophical doctrines to the truths of the Bible. While Nestorianism and Arianism disappeared from within the Empire, the simpler concept that Christ was only a man was adopted by the German tribes, since Nestorius and Arius were energetic at sending missionaries to them.
As a result, when these German tribes conquered the Western Empire, their heretical views made it harder for them to join in fellowship with the Christians who accepted the Creed. This had a highly disruptive effect on later western history.