17 Let the new people, therefore, who are called to an eternal inheritance, use the word of the New Testament, and say, “Our Father who art in heaven,” i.e. in the holy and the just. For God is not contained in space. For the heavens are indeed the higher material bodies of the world, but yet material, and therefore cannot exist except in some definite place; but if God's place is believed to be in the heavens, as meaning the higher parts of the world, the birds are of greater value than we, for their life is nearer to God. But it is not written, The Lord is near unto tall men, or unto those who dwell on mountains; but it is written, “The Lord is near unto them that are of a broken heart,” which refers rather to humility. But as a sinner is called earth, when it is said to him, “Earth you are, and unto earth shall you return;” so, on the other hand, a righteous man may be called heaven. For it is said to the righteous, “For the temple of God is holy, which temple you are.” And therefore, if God dwells in His temple, and the saints are His temple, the expression “which art in heaven” is rightly used in the sense, which art in the saints. And most suitable is such a similitude, so that spiritually there may be seen to be as great a difference between the righteous and sinners, as there is materially between heaven and earth.
18. And for the purpose of showing this, when we stand at prayer, we turn to the east, whence the heaven rises: not as if God also were dwelling there, in the sense that He who is everywhere present, not as occupying space, but by the power of His majesty, had forsaken the other parts of the world; but in order that the mind may be admonished to turn to a more excellent nature, i.e. to God, when its own body, which is earthly, is turned to a more excellent body, i.e. to a heavenly one. It is also suitable for the different stages of religion, and expedient in the highest degree, that in the minds of all, both small and great, there should be cherished worthy conceptions of God. And therefore, as regards those who as yet are taken up with the beauties that are seen, and cannot think of anything incorporeal, inasmuch as they must necessarily prefer heaven to earth, their opinion is more tolerable, if they believe God, whom as yet they think of after a corporeal fashion, to be in heaven rather than upon earth: so that when at any future time they have learned that the dignity of the soul exceeds even a celestial body, they may seek Him in the soul rather than in a celestial body even; and when they have learned how great a distance there is between the souls of sinners and of the righteous, just as they did not venture, when as yet they were wise only after a carnal fashion, to place Him on earth, but in heaven, so afterwards with better faith or intelligence they may seek Him again in the souls of the righteous rather than in those of sinners. Hence, when it is said, “Our Father which art in heaven,” it is rightly understood to mean in the hearts of the righteous, as it were in His holy temple. And at the same time, in such a way that he who prays wishes Him whom he invokes to dwell in himself also; and when he strives after this, practises righteousness—a kind of service by which God is attracted to dwell in the soul.
19. Let us see now what things are to be prayed for. For it has been stated who it is that is prayed to, and where He dwells. First of all, then, of those things which are prayed for comes this petition, “Hallowed be Your name.” And this is prayed for, not as if the name of God were not holy already, but that it may be held holy by men; i.e., that God may so become known to them, that they shall reckon nothing more holy, and which they are more afraid of offending. For, because it is said, “In Judah is God known; His name is great in Israel,” we are not to understand the statement in this way, as if God were less in one place, greater in another; but there His name is great, where He is named according to the greatness of His majesty. And so there His name is said to be holy, where He is named with veneration and the fear of offending Him. And this is what is now going on, while the gospel, by becoming known everywhere throughout the different nations, commends the name of the one God by means of the administration of His Son.
Source: Our Lord's Sermon on the Mount (New Advent)