St. Augustine on the Usefulness of Heretics

In Augustine's exposition of Psalm 55 there is a brilliant defense of the necessity of heretics within the church. The Apostle Paul writes in 1 Corinthians 11:19 For there must be also heresies among you, that they which are approved may be made manifest among you. Heretics and dissensions try, prove, and purge the church. As Augustine writes, 

"For many things lay hid in the Scriptures: and when heretics had been cut off, with questions they troubled the Church of God: then those things were opened which lay hid, and the will of God was understood."

Without heretics coming along and causing dissension with their false teachings many truths would have remain hidden because there was no need to dredge them up. It is like a snow globe which unless it is shaken remains peaceful and quiet. But once it is shaken all the snow floats around the water and perhaps even a sweet melody is played once the key is turned.

21. And His heart has drawn near Psalm 54:22. Of whom do we understand it, except of Him, by the anger of whom they have been divided? How has his heart drawn near? In such sort, that we may understand His will. For by heretics has been vindicated the Catholic Church, and by those that think evil have been proved those that think well. For many things lay hid in the Scriptures: and when heretics had been cut off, with questions they troubled the Church of God: then those things were opened which lay hid, and the will of God was understood. Thence is said in another Psalm, In order that they might be excluded that have been proved with silver. For let them be excluded, He has said, let them come forth, let them appear. Whence even in silver-working men are called excluders, that is, pressers out of form from the sort of confusion of the lump. Therefore many men that could understand and expound the Scriptures very excellently, were hidden among the people of God: but they did not declare the solution of difficult questions, when no reviler again urged them. For was the Trinity perfectly treated of before the Arians snarled thereat? Was repentance perfectly treated of before the Novatians opposed? So not perfectly of Baptism was it treated, before rebaptizers removed outside contradicted; nor of the very oneness of Christ were the doctrines clearly stated which have been stated, save after that this separation began to press upon the weak: in order that they that knew how to treat of and solve these questions (lest the weak should perish vexed with the questions of the ungodly), by their discourses and disputations should bring out unto open day the dark things of the Law.. ..This obscure sense see in what manner the Apostle brings out into light; It is needful, he says, that also heresies there be, in order that men proved may be made manifest among you. 1 Corinthians 11:19 What is men proved? Proved with silver, proved with the word. What is may be made manifest? May be brought out. Wherefore this? Because of heretics. So therefore these also have been divided because of the anger of His countenance, and His heart has drawn near.

 http://www.newadvent.org/fathers/1801055.htm