14 Now because by reason of those daily sins of which I have spoken, it is necessary for you to say, in that daily prayer of cleansing as it were, “Forgive us our debts, as we also forgive our debtors;” what will you do? You have enemies. For who can live on this earth without them? Take heed to yourselves, love them. In no way can your enemy so hurt you by his violence, as you hurt yourself if you love him not. For he may injure your estate, or flocks, or house, or your man-servant, or your maid-servant, or your son, or your wife; or at most, if such power be given him, your body.
But can he injure your soul, as you can yourself? Reach forward, dearly beloved, I beseech you, to this perfection. But have I given you this power? He only has given it to whom you say, Your will be done as in heaven so in earth. Yet let it not seem impossible to you. I know, I have known by experience, that there are Christian men who do love their enemies. If it seem to you impossible, you will not do it. Believe then first that it can be done, and pray that the will of God may be done in you.
For what good can your neighbour's ill do you? If he had no ill, he would not even be your enemy. Wish him well then, that he may end his ill, and he will be your enemy no longer. For it is not the human nature in him that is at enmity with you, but his sin. Is he therefore your enemy, because he has a soul and body? In this he is as you are: you have a soul, and so has he: you have a body, and so has he. He is of the same substance as you are; you were made both out of the same earth, and quickened by the same Lord.
In all this he is as you are. Acknowledge in him then your brother. The first pair, Adam and Eve, were our parents; the one our father, the other our mother; and therefore we are brethren. But let us leave the consideration of our first origin. God is our Father, the Church our Mother, and therefore are we brethren. But you will say, my enemy is a heathen, a Jew, a heretic, of whom I spoke some time ago on the words, “Your will be done as in heaven so in earth.” O Church, your enemy is the heathen, the Jew, the heretic; he is the earth.
If you are heaven, call on your Father which is in heaven, and pray for your enemies: for so was Saul an enemy of the Church; thus was prayer made for him, and he became her friend. He not only ceased from being her persecutor, but he laboured to be her helper. And yet, to say the truth, prayer was made against him; but against his malice, not against his nature. So let your prayer be against the malice of your enemy, that it may die, and he may live. For if your enemy were dead, you have lost it might seem an enemy, yet have you not found a friend. But if his malice die, you have at once lost an enemy and found a friend.
Source: Sermons on Selected Lessons of the New Testament (New Advent)