“the conflict between Cain and Abel displayed the hostility between the two cities themselves, the City of God and the city of men. Thus the wicked fight among themselves; and likewise the wicked fight against the good and the good against the wicked. But the good, if they have reached perfect goodness, cannot fight among themselves. However, while they are on their way towards the perfection they have not yet attained, there may be fighting among them…” [The City of God, 15.5]
Christ did not come to bring peace but a sword. This sword does not characterize the relationships between Christians but that between the world and the Church. The Apostle John wrote, “Do not be surprised, brothers, that the world hates you,” (1 John 3:13) and he rooted this in the conflict between Cain and Abel (1 John 3:12). In other words, the murder of Abel was more than a single, isolated sin. God intended it as the paradigm of the hostility that the world would have toward the Church during its pilgrimage toward its heavenly kingdom.
First John proceeds to speak of the love Christians have for one another, something which was surely in the back of Augustine’s mind. “We know that we have passed out of death into life, because we love the brothers. Whoever does not love abides in death.” (1 John 3:14) Whereas the world is characterized by enmity within itself and toward the Church, the Church is characterized as love within itself. Of course, it is not called to be in spiritual communion with the world (1 John 2:15, “Do not love the world of the things in the world.”), so this estrangement is unavoidable.