For “he who digs a pit for his neighbor, falls into it.” Let us then not plot against others, lest we injure ourselves. When we supplant the reputation of others, let us consider that we injure ourselves, it is against ourselves we plot. For perchance with men we do him harm, if we have power, but we injure ourselves in the sight of God, by provoking Him against us. Let us not then harm ourselves. For as we injure ourselves when we injure our neighbors, so by benefiting them we benefit ourselves. If then your enemy harm you, he has benefited you if you are wise, and so requite him not with the same things, but even do him good. But the blow, you say, remains severe. Consider then that thou dost not benefit, but punishest him, and benefitest yourself, and quickly you will come to do him good. What then? Shall we act from this motive? We ought not to act on this motive, but if your heart will not hear other reason, induce it, says he, even by this, and you will quickly persuade it to dismiss its enmity, and wilt for the future do good to your enemy as to a friend, and wilt obtain the good things which are to come, to which God grant that we may all attain in Christ Jesus. Amen.
Source: Homilies on Philippians (New Advent)