25 The fourth petition is, “Give us this day our daily bread.” Daily bread is put either for all those things which meet the wants of this life, in reference to which He says in His teaching, “Take no thought for the morrow:” so that on this account there is added, “Give us this day:” or, it is put for the sacrament of the body of Christ, which we daily receive: or, for the spiritual food, of which the same Lord says, “Labour for the meat which perishes not;” and again, “I am the bread of life, which came down from heaven.” But which of these three views is the more probable, is a question for consideration. For perhaps some one may wonder why we should pray that we may obtain the things which are necessary for this life—such, for instance, as food and clothing—when the Lord Himself says, “Be not anxious what you shall eat, or what you shall put on.” Can any one not be anxious for a thing which he prays that he may obtain; when prayer is to be offered with so great earnestness of mind, that to this refers all that has been said about shutting our closets, and also the command, “Seek first the kingdom of God, and His righteousness; and all these things shall be added unto you”? Certainly He does not say, Seek first the kingdom of God, and then seek those other things; but “all these things,” says He, “shall be added unto you,” that is to say, even though you are not seeking them. But I know not whether it can be found out, how one is rightly said not to seek what he most earnestly pleads with God that he may receive.
26. But with respect to the sacrament of the Lord's body (in order that they may not start a question, who, the most of them being in Eastern parts, do not partake of the Lord's supper daily, while this bread is called daily bread: in order, therefore, that they may be silent, and not defend their way of thinking about this matter even by the very authority of the Church, because they do such things without scandal, and are not prevented from doing them by those who preside over their churches, and when they do not obey are not condemned; whence it is proved that this is not understood as daily bread in these parts: for, if this were the case, they would be charged with the commission of a great sin, who do not on that account receive it daily; but, as has been said, not to argue at all to any extent from the case of such parties), this consideration at least ought to occur to those who reflect, that we have received a rule for prayer from the Lord, which we ought not to transgress, either by adding or omitting anything. And since this is the case, who is there who would venture to say that we ought only once to use the Lord's Prayer, or at least that, even if we have used it a second or a third time before the hour at which we partake of the Lord's body, afterwards we are assuredly not so to pray during the remaining hours of the day? For we shall no longer be able to say, “Give us this day,” respecting what we have already received; or every one will be able to compel us to celebrate that sacrament at the very last hour of the day.
27. It remains, therefore, that we should understand the daily bread as spiritual, that is to say, divine precepts, which we ought daily to meditate and to labour after. For just with respect to these the Lord says, “Labour for the meat which perishes not.” That food, moreover, is called daily food at present, so long as this temporal life is measured off by means of days that depart and return. And, in truth, so long as the desire of the soul is directed by turns, now to what is higher, now to what is lower, i.e. now to spiritual things, now to carnal, as is the case with him who at one time is nourished with food, at another time suffers hunger; bread is daily necessary, in order that the hungry man may be recruited, and he who is falling down may be raised up. As, therefore, our body in this life, that is to say, before that great change, is recruited with food, because it feels loss; so may the soul also, since by means of temporal desires it sustains as it were a loss in its striving after God, be reinvigorated by the food of the precepts. Moreover, it is said, “Give us this day,” as long as it is called today, i.e. in this temporal life. For we shall be so abundantly provided with spiritual food after this life unto eternity, that it will not then be called daily bread; because there the flight of time, which causes days to succeed days, whence it may be called today, will not exist. But as it is said, “Today, if you will hear His voice,” which the apostle interprets in the Epistle to the Hebrews, As long as it is called today; so here also the expression is to be understood, “Give us this day.” But if any one wishes to understand the sentence before us also of food necessary for the body, or of the sacrament of the Lord's body, we must take all three meanings conjointly; that is to say, that we are to ask for all at once as daily bread, both the bread necessary for the body, and the visible hallowed bread, and the invisible bread of the word of God.
Source: Our Lord's Sermon on the Mount (New Advent)