11 What then do our adversaries say? “If,” says one, “I shall discover a lie, surely you will not then believe it all; and such I have discovered.” Let us see: I will reckon up the generations; for by their slanderous cavillings they invite and bring us to this. Yes, if we live religiously, if we believe Christ, if we do not desire to fly out of the nest before the time, they only bring us to this— to the knowledge of mysteries. Mark then, holy brethren, the usefulness of heretics; their usefulness, that is, in respect of the designs of God, who makes a good use even of those that are bad; whereas, as regards themselves, the fruit of their own designs is rendered to them, and not that good which God brings out of them.
Just as in the case of Judas; what great good did he! By the Lord's Passion all nations are saved; but that the Lord might suffer, Judas betrayed Him. God then both delivers the nations by the Passion of His Son, and punishes Judas for his own wickedness. For the mysteries which lie hidden in Scripture, no one who is content with the simplicity of the faith would curiously sift them, and therefore as no one would sift them, no one would discover them but for cavillers who force us.
For when heretics cavil, the little ones are disturbed; when disturbed, they make search, and their search is, so to say, a beating of the head at the mother's breasts, that they may yield as much milk as is sufficient for these little ones. They search then, because they are troubled; but they who know and have learned these things, because they have investigated them, and God has opened to their knocking, they in their turn open to those who are in trouble. And so it happens that heretics serve usefully for the discovery of the truth, while they cavil to seduce men into error.
For with less carefulness would truth be sought out, if it had not lying adversaries; “For there must be also heresies among you,” and as though we should enquire the cause, he immediately subjoined, “that they which are approved may be made manifest among you.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)