Miscellanies 1.17.86

But the counsels and activities of the apostates too, being partial, arise from a bad disposition, like bodily diseases; but they are steered by universal providence to a healthy end, even if the cause produces disease. At any rate, the greatest thing of divine providence is not to allow the vice that sprang from a voluntary apostasy to remain useless and unprofitable, nor to become harmful in every respect. For it is the work of divine wisdom, virtue, and power not only to do good (for this, so to speak, is God's nature, as it is fire's to warm and light's to give light), but especially this: to bring about some good and useful end through the evils devised by some, and to use profitably what seems bad, as in the case of the martyrdom that comes from temptation.