Of course it is absolutely sinful to have any private enmities. Not the bitterest personal assaults upon us should induce us to retaliate. We must do good for evil, "love those who hate, bless those who curse us, and pray for those who despitefully use us." It is only when it is impossible at once to be kind to them, and give glory to God, that we may cease to act kindly towards them.
--Blessed John Henry Newman, "Jewish Zeal, a Pattern for Christians", Parochial and Plain Sermons, Vol. 3