"What is perfection in love? To love even one's enemies, and to love them to the degree that they may be brothers...Love your enemies in such a way that you wish them to be brothers; love your enemies in such a way that they be brought into your fellowship. For that is how he loved who, as he hung on the cross, said, 'Father, forgive them, because they do not know what they are doing'...Many of them believed, and the shedding of Christ's blood was forgiven them. At first, when they were raging, they poured it out; then, when they believed, they drank it."
--St. Augustine, First Homily on the First Epistle of John
From his sermon on Matthew 6:
"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."
Love of our enemies is the only God-like love available to us, because it is unmerited, even as God loved us to the death while we were yet his enemies. Only this kind of self-sacrificing, cross-embracing love changes the world.