Chapter 80.
175 Petilianus said: "Lastly, what is the justification of persecution? I ask you, you wretched men, if it so be that you think that your sin rests on any authority of law."
176. Augustine answered: He who sins, sins not on the authority of the law, but against the authority of the law. But since you ask what is the justification of persecution, I ask you in turn whose voice it is that says in the psalm, "Whoso privily slanders his neighbor, him will I cut off." Seek therefore the reason or the measure of the persecution, and do not display your gross ignorance by finding fault in general terms with those who persecute the unrighteous.
Source: Answer to Letters of Petilian, Bishop of Cirta (New Advent)