11. Often, too, divine providence permits even good men to be driven from the congregation of Christ by the turbulent seditions of carnal men. When for the sake of the peace of the Church they patiently endure that insult or injury, and attempt no novelties in the way of heresy or schism, they will teach men how God is to be served with a true disposition and with great and sincere charity. The intention of such men is to return when the tumult has subsided. But if that is not permitted because the storm continues or because a fiercer one might be stirred up by their return, they hold fast to their purpose to look to the good even of those responsible for the tumults and commotions that drove them out. They form no separate conventicles of their own, but defend to the death and assist by their testimony the faith which they know is preached in the Catholic Church. These the Father who seeth in secret crowns secretly. It appears that this is a rare kind of Christian, but examples are not lacking. Indeed, there are more than can be believed. So divine providence uses all kinds of men as examples for the oversight of souls and for the building up of his spiritual people.
15. It has been truly said: “There must be many heresies, that they which are approved may be made manifest among you” (I Cor. 11:19 ). Let us also make use of that gift of divine providence. Men become heretics who would have no less held wrong opinions even within the Church. Now that they are outside they do us more good, not by teaching the truth, for they do not know it, but by provoking carnal Catholics to seek the truth and spiritual Catholics to expound it. There are in the Holy Church innumerable men approved by God, but they do not become manifest among us so long as we are delighted with the darkness of our ignorance and prefer to sleep rather than to behold the light of truth. So, many are awakened from sleep by the heretics, so that they may see God’s light and be glad. Let us therefore use even heretics, not to approve their errors but to assert the Catholic discipline against their wiles and to become more vigilant and cautious even if we candot recall them to salvation.
(De vera religione, in Augustine: Earlier Writings, translated by John H. S. Burleigh [Philadelphia: The Westminster Press, 1953], 231 and 233.)