Two perceptive comments on heretics from Vincent of Lerins. The one states the principle, the other supplies an illustration of it:
The reason is clearer than day why Divine Providence sometimes permits certain doctors of the Churches to preach new doctrines--"That the Lord your God may try you;" he says. And assuredly it is a great trial when one whom thou believest to be a prophet, a disciple of prophets, a doctor and defender of the truth, whom thou hast folded to thy breast with the utmost veneration and love, when such a one of a sudden secretly and furtively brings in noxious errrors, which thou canst neither quickly detect, being held by the prestige of former authority, nor lightly think it right to condemn, being prevented by affection for thine old master.
For to take first a very recent and very plain case: what sort of trial, think we, was that which the Church had experience of the other day, when that unhappy Nestorius, all at once metamorphosed from a sheep into a wolf, began to make havoc of the flock of Christ, while as yet a large proportion of those whom he was devouring believed him to be a sheep, and consequently were the more exposed to his attacks? For who would readily suppose him to be in error, who was known to have been elected by the high choice of the Emperor, and to be held in the greatest esteem by the priesthood? who would readily suppose him to be in error, who, greatly beloved by the holy brethren, and in high favor with the populace, expounded the Scriptures in public daily, and confuted the pestilent errors both of Jews and Heathens? Who could choose but believe that his teaching was Orthodox, his preaching Orthodox, his belief Orthodox, who, that he might open the way to one heresy of his own, was zealously inveighing against the blasphemies of all heresies? But this was the very thing which Moses says: "The Lord your God doth try you that He may know whether you love Him or not."
Vincent of Lerins, Commonitorium, Extracts from Chapters X & XI