"You have heard that it was said, 'You shall love your neighbor and hate your enemy.' But I say to you, Love your enemies and pray for those who persecute you, so that you may be sons of your Father who is in heaven for he makes his sun rise on the evil and on the good, and sends rain on the just and on the unjust. For if you love those who love you, what reward have you? Do not even the tax collectors do the same? And if you salute only your brethren? Do not even the Gentiles do the same? You, therefore, must be perfect, as your heavenly Father is perfect."
Matthew 5: 43-48
"We should, on the contrary, forget the offenses of our enemy, even though he neither repents nor amends, in imitation of Christ who prayed for those who crucified Him, and who, far from repenting, mocked Him... For the height of perfection is to love our enemies, and to pray for them as did the Lord Jesus."
St. Thomas Aquinas as quoted in Sermon in a Sentence: Volume 5
"If we could read the secret history of our enemies, we should find in
each man's life sorrow and suffering enough to disarm all hostility."
Henry Wadsworth Longfellow