If we speak, however, of what are called “corporeal” and “external” evils—which are improperly so termed—then it may be granted that there are occasions when some of these have been called into existence by God, in order that by their means the conversion of certain individuals might be effected. And what absurdity would follow from such a course? For as, if we should hear those sufferings improperly termed “evils” which are inflicted by fathers, and instructors, and pedagogues upon those who are under their care, or upon patients who are operated upon or cauterized by the surgeons in order to effect a cure, we were to say that a father was ill-treating his son, or pedagogues and instructors their pupils, or physicians their patients, no blame would be laid upon the operators or chastisers; so, in the same way, if God is said to bring upon men such evils for the conversion and cure of those who need this discipline, there would be no absurdity in the view, nor would “evils come down from the Lord upon the gates of Jerusalem,” — which evils consist of the punishments inflicted upon the Israelites by their enemies with a view to their conversion; nor would one visit “with a rod the transgressions of those who forsake the law of the Lord, and their iniquities with stripes;” nor could it be said, “You have coals of fire to set upon them; they shall be to you a help.” In the same way also we explain the expressions, “I, who make peace, and create evil;” for He calls into existence “corporeal” or “external” evils, while purifying and training those who would not be disciplined by the word and sound doctrine. This, then, is our answer to the question, “How is it that God created evil?”
Source: Against Celsus (New Advent)