The Catholic Church in North Africa was in crisis at the beginning of the fourth century. The Roman emperor Diocletian had persecuted the Christians, and many bishops and priests had collaborated with the regime. Priests had turned over Christian believers to the pagan magistrates. Bishops had surrendered Holy Scriptures to be burned in the public square. An air of corruption and lewdness hung over the church.
Two rival reform movements arose to restore the integrity of Catholicism. Those in the first movement, the Donatists, believed the church needed to purify itself and return to its core identity.
The mission of the church, in the Donatist view, was to provide a holy alternative to a unclean world. The Donatists wanted to purge the traitors from the priesthood.
After they pruned their membership, the Donatists wanted to close ranks to create a community of committed believers. They would separate themselves from impurity, re-establish their core principles and defend them against the hostile forces.
The Donatists believed that, in those hard times, the first job was to defend Christian law so it wouldn't be diluted by compromise. With this defensive posture, the Donatists would at least build a sturdy ark for all those who wanted to be Christian.
This Donatist tendency — to close ranks and return defensively to first principles — can be seen today whenever a movement faces a crisis. Modern-day Donatists emerge after every Republican defeat: conservatives who think the main task is to purge and purify. There are modern-day Donatists in humanities departments, who pull in as they lose relevance on campus.
You can see them in the waning union movement: people who double down on history and their self-conscious traditions. You can see them in the current Roman Catholic Church, which feels besieged in a hostile world. You can identify the modern-day Donatists because they feel history is flowing away from them, and when they gossip it's always about intra-community rivalries that nobody outside their world could possibly care about.
In the fourth century, another revival movement arose, embraced by Augustine, who was Bishop of Hippo. The problem with the Donatists, Augustine argued, is that they are too static. They try to seal off an ark to ride out the storm, but they end up sealing themselves in. They cut themselves off from new circumstances and growth.
Augustine, as his magisterial biographer Peter Brown puts it, "was deeply preoccupied by the idea of the basic unity of the human race." He reacted against any effort to divide people between those within the church and those permanently outside.
He wanted the church to go on offense and swallow the world. This would involve swallowing impurities as well as purities. It would mean putting to use those who are imperfect. This was the price to be paid if you wanted an active church coexisting with sinners, disciplining and rebuking them.