83 You perceive what kind of teacher must be found for the preservation or teaching of this gift, and we can find him, if you assist by unanimity, if you forgive one another should any one think himself injured by another. For it is not the only kind of justice, not to injure him who has not injured us, but also to forgive him who has most injured us. We are often injured by the fraud of another, by the guile of a neighbour; do we consider it a mark of virtue, to avenge guile by guile, or to repay fraud by fraud?
For if justice is a virtue it should be free from offense, and should not repel wickedness by wickedness. For what virtue is it that the same thing should be done by you which you yourself punish in another? That is the spreading of wickedness not its punishment, for it makes no difference whom one injures, whether a just man or an unjust, seeing one ought not to injure anyone. Nor does it make any difference in what way one bears ill will, whether from a desire of revenging oneself, or from a wish to injure, since in neither case is ill will free from blame.
For to bear ill will is the same thing as to be unjust, and so it is said to you: “Bear not ill will among those that bear ill will, and emulate not those that do unrighteousness;” and above; “I have hated the congregation of them that bear ill will.” He clearly comprehends all and makes no exception, he lays hold of ill will and asks not the cause.
Source: Letters (New Advent)