20 In the next place there follows, “Your kingdom come.” Just as the Lord Himself teaches in the Gospel that the day of judgment will take place at the very time when the gospel shall have been preached among all nations: a thing which belongs to the hallowing of God's name. For here also the expression “Your kingdom come” is not used in such a way as if God were not now reigning. But some one perhaps might say the expression “come” meant upon earth; as if, indeed, He were not even now really reigning upon earth, and had not always reigned upon it from the foundation of the world. “Come,” therefore, is to be understood in the sense of “manifested to men.” For in the same way also as a light which is present is absent to the blind, and to those who shut their eyes; so the kingdom of God, though it never departs from the earth, is yet absent to those who are ignorant of it. But no one will be allowed to be ignorant of the kingdom of God, when His Only-begotten shall come from heaven, not only in a way to be apprehended by the understanding, but also visibly in the person of the Divine Man, in order to judge the quick and the dead. And after that judgment, i.e. when the process of distinguishing and separating the righteous from the unrighteous has taken place, God will so dwell in the righteous, that there will be no need for any one being taught by man, but all will be, as it is written, “taught of God.” Then will the blessed life in all its parts be perfected in the saints unto eternity, just as now the most holy and blessed heavenly angels are wise and blessed, from the fact that God alone is their light; because the Lord has promised this also to His own: “In the resurrection,” says He, “they will be as the angels in heaven.”
21. And therefore, after that petition where we say, “Your kingdom come,” there follows, “Your will be done, as in heaven so in earth:” i.e., just as Your will is in the angels who are in heaven, so that they wholly cleave to You, and thoroughly enjoy You, no error beclouding their wisdom, no misery hindering their blessedness; so let it be done in Your saints who are on earth, and made from the earth, so far as the body is concerned, and who, although it is into a heavenly habitation and exchange, are yet to be taken from the earth. To this there is a reference also in that doxology of the angels, “Glory to God in the highest, and on earth peace to men of goodwill:” so that when our goodwill has gone before, which follows Him that calls, the will of God is perfected in us, as it is in the heavenly angels; so that no antagonism stands in the way of our blessedness: and this is peace. “Your will be done” is also rightly understood in the sense of, Let obedience be rendered to Your precepts: “as in heaven so on earth,” i.e. as by the angels so by men. For, that the will of God is done when His precepts are obeyed, the Lord Himself says, when He affirms, “My meat is to do the will of Him that sent me;” and often, “I came, not to do my own will, but the will of Him that sent me;” and when He says, “Behold my mother and my brethren! For whosoever shall do the will of God, the same is my brother, and sister, and mother.” And therefore, in those at least who do the will of God, the will of God is accomplished; not because they cause God to will, but because they do what He wills, i.e. they do according to His will.
22. There is also that other interpretation, “Your will be done as in heaven so on earth,”— as in the holy and just, so also in sinners. And this, besides, may be understood in two ways: either that we should pray even for our enemies (for what else are they to be reckoned, in spite of whose will the Christian and Catholic name still spreads?), so that it is said, “Your will be done as in heaven so on earth,”— as if the meaning were, As the righteous do Your will, in like manner let sinners also do it, so that they may be converted unto You; or in this sense, “Let Your will be done as in heaven so on earth,” so that every one may get his own; which will take place at the last judgment, the righteous being requited with a reward, sinners with condemnation— when the sheep shall be separated from the goats.
23. That other interpretation also is not absurd, nay, it is thoroughly accordant with both our faith and hope, that we are to take heaven and earth in the sense of spirit and flesh. And since the apostle says, “With the mind I myself serve the law of God, but with the flesh the law of sin,” we see that the will of God is done in the mind, i.e. in the spirit. But when death shall have been swallowed up in victory, and this mortal shall have put on immortality, which will happen at the resurrection of the flesh, and at that change which is promised to the righteous, according to the prediction of the same apostle, let the will of God be done on earth, as it is in heaven; i.e., in such a way that, in like manner as the spirit does not resist God, but follows and does His will, so the body also may not resist the spirit or soul, which at present is harassed by the weakness of the body, and is prone to fleshly habit: and this will be an element of the perfect peace in the life eternal, that not only will the will be present with us, but also the performance of that which is good. “For to will,” says he, “is present with me; but how to perform that which is good I find not:” for not yet in earth as in heaven, i.e. not yet in the flesh as in the spirit, is the will of God done. For even in our misery the will of God is done, when we suffer those things through the flesh which are due to us in virtue of our mortality, which our nature has deserved because of its sin. But we are to pray for this, that the will of God may be done as in heaven so in earth; that in like manner as with the heart we delight in the law after the inward man, so also, when the change in our body has taken place, no part of us may, on account of earthly griefs or pleasures, stand opposed to this our delight.
24. Nor is that view inconsistent with truth, that we are to understand the words, “Your will be done as in heaven so in earth,” as in our Lord Jesus Christ Himself, so also in the Church: as if one were to say, As in the man who fulfilled the will of the Father, so also in the woman who is betrothed to him. For heaven and earth are suitably understood as if they were man and wife; since the earth is fruitful from the heaven fertilizing it.
Source: Our Lord's Sermon on the Mount (New Advent)