From Augustine’s letter to Vincentius:

16. You now see therefore, I suppose, that the thing to be considered when any one is coerced, is not the mere fact of the coercion, but the nature of that to which he is coerced, whether it be good or bad: not that any one can be good in spite of his own will, but that, through fear of suffering what he does not desire, he either renounces his hostile prejudices, or is compelled to examine truth of which he had been contentedly ignorant; and under the influence of this fear repudiates the error which he was wont to defend, or seeks the truth of which he formerly knew nothing, and now willingly holds what he formerly rejected. […] For originally my opinion was, that no one should be coerced into the unity of Christ, that we must act only by words, fight only by arguments, and prevail by force of reason, lest we should have those whom we knew as avowed heretics feigning themselves to be Catholics. But this opinion of mine was overcome not by the words of those who controverted it, but by the conclusive instances to which they could point. For, in the first place, there was set over against my opinion my own town, which, although it was once wholly on the side of Donatus, was brought over to the Catholic unity by fear of the imperial edicts, but which we now see filled with such detestation of your ruinous perversity, that it would scarcely be believed that it had ever been involved in your error.