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.
How great the reward of mercy which is received into the privilege of divine adoption! For it follows, And ye shall be the sons of the Highest (Ps. 82:6.). Follow then mercy, that ye may obtain grace. Widely spread is the mercy of God; He pours His rain upon the unthankful, the fruitful earth refuses not its increase to the evil. Hence it follows, For he is kind to the unthankful, and to the evil.