78 Then, as to the statement which follows, “that you may be the children of your Father which is in heaven,” it is to be understood according to that rule in virtue of which John also says, “He gave them power to become the sons of God.” For one is a Son by nature, who knows nothing at all of sin; but we, by receiving power, are made sons, in as far as we perform those things which are commanded us by Him. And hence the apostolic teaching gives the name of adoption to that by which we are called to an eternal inheritance, that we may be joint-heirs with Christ. We are therefore made sons by a spiritual regeneration, and we are adopted into the kingdom of God, not as aliens, but as being made and created by Him: so that it is one benefit, His having brought us into being through His omnipotence, when before we were nothing; another, His having adopted us, so that, as being sons, we might enjoy along with Him eternal life for our participation. Therefore He does not say, Do those things, because you are sons; but, Do those things, that you may be sons.
79. But when He calls us to this by the Only-begotten Himself, He calls us to His own likeness. For He, as is said in what follows, “makes His sun to rise on the evil and on the good, and sends rain on the just and on the unjust.” Whether you are to understand His sun as being not that which is visible to the fleshly eyes, but that wisdom of which it is said, “She is the brightness of the everlasting light;” of which it is also said, “The Sun of righteousness has arisen upon me;” and again, “But unto you that fear the name of the Lord shall the Sun of righteousness arise:” so that you would also understand the rain as being the watering with the doctrine of truth, because Christ has appeared to the good and the evil, and is preached to the good and the evil. Or whether you choose rather to understand that sun which is set forth before the bodily eyes not only of men, but also of cattle; and that rain by which the fruits are brought forth, which have been given for the refreshment of the body, which I think is the more probable interpretation: so that that spiritual sun does not rise except on the good and holy; for it is this very thing which the wicked bewail in that book which is called the Wisdom of Solomon, “And the sun rose not upon us:” and that spiritual rain does not water any except the good; for the wicked were meant by the vineyard of which it is said, “I will also command my clouds that they rain no rain upon it.” But whether you understand the one or the other, it takes place by the great goodness of God, which we are commanded to imitate, if we wish to be the children of God. For who is there so ungrateful as not to feel how great the comfort, so far as this life is concerned, which that visible light and the material rain bring? And this comfort we see bestowed in this life alike upon the righteous and upon sinners in common. But He does not say, “who makes the sun to rise on the evil and on the good;” but He has added the word “His,” i.e. which He Himself made and established, and for the making of which He took nothing from any one, as it is written in Genesis respecting all the luminaries; and He can properly say that all the things which He has created out of nothing are His own: so that we are hence admonished with how great liberality we ought, according to His precept, to give to our enemies those things which we have not created, but have received from His gifts.
80. But who can either be prepared to bear injuries from the weak, in as far as it is profitable for their salvation; and to choose rather to suffer more injustice from another than to repay what he has suffered; to give to every one that asks anything from him, either what he asks, if it is in his possession, and if it can rightly be given, or good advice, or to manifest a benevolent disposition, and not to turn away from him who desires to borrow; to love his enemies, to do good to those who hate him, to pray for those who persecute him—who, I say, does these things, but the man who is fully and perfectly merciful? And with that counsel misery is avoided, by the assistance of Him who says, “I desire mercy, and not sacrifice.” “Blessed,” therefore, “are the merciful: for they shall obtain mercy.” But now I think it will be more convenient, that at this point the reader, fatigued with so long a volume, should breathe a little, and recruit himself for considering what remains in another book.
Source: Our Lord's Sermon on the Mount (New Advent)