84 But what better pattern can there be than that of Divine justice? For the Son of God says: “Love your enemies;” and again: “Pray for those that persecute you and speak against you.” So far does He remove the desire of vengeance from the perfect that He commands charity towards those who injure them. And since He had said in the Old Testament: “Vengeance is Mine, I will repay.” He says in the Gospel, that we are to pray for those who have injured us, that He Who has said that He will avenge, may not do so; for it is His will to pardon at your desire with which according to His promise He agrees. But if you seek for you know that the unjust is more severely punished by his own convictions than by judicial severity.
Source: Letters (New Advent)