Ambrose of Milan
Catena Aurea · Lukasevangelium 6
For if ye love them which love you, what thank have ye? for sinners also love those that love them. And if ye do good to them which do good to you, what thank have ye? for sinners also do even the same. And if ye lend to them of whom ye hope to receive, what thank have ye? for sinners also lend to sinners, to receive as much again. But love ye your enemies, and do good, and lend, hoping for nothing again; and your reward shall be great, and ye shall be the children of the Highest: for he is kind unto the unthankful and to the evil. Be ye therefore merciful, as your Father also is merciful.
Now philosophy seems to divide justice into three parts; one towards God, which is called piety; another towards our parents, or the rest of mankind; a third to the dead, that the proper rites may be performed. But the Lord Jesus passing beyond the oracle of the law, and the heights of prophecy, extended the duties of piety to those also who have injured us, adding, But love your enemies.