5 Then He speaks even the very words of the prayer.
“When ye pray,” says He, “use no vain repetitions, even as the heathen do.”
You see that when He was discoursing of almsgiving, He removed only that mischief which comes of vainglory, and added nothing more; neither did He say whence one should give alms; as from honest labor, and not from rapine nor covetousness: this being abundantly acknowledged among all. And also before that, He had thoroughly cleared up this point, when He blessed them “that hunger after righteousness.”
But touching prayer, He adds somewhat over and above; “not to use vain repetitions.” And as there He derides the hypocrites, so here the heathen; shaming the hearer everywhere most of all by the vileness of the persons. For since this, in most cases, is especially biting and stinging, I mean our appearing to be likened to outcast persons; by this topic He dissuades them; calling frivolousness, here, by the name of “vain repetition:” as when we ask of God things unsuitable, kingdoms, and glory, and to get the better of enemies, and abundance of wealth, and in general what does not at all concern us.
“For He knows,” says He, “what things you have need of.”
And herewith He seems to me to command in this place, that neither should we make our prayers long; long, I mean, not in time, but in the number and length of the things mentioned. For perseverance indeed in the same requests is our duty: His word being, “continuing instant in prayer.”
And He Himself too, by that example of the widow, who prevailed with the pitiless and cruel ruler, by the continuance of her intercession; and by that of the friend, who came late at night time, and roused the sleeper from his bed, not for his friendship's, but for his importunity's sake; what did He, but lay down a law, that all should continually make supplication unto Him? He does not however bid us compose a prayer of ten thousand clauses, and so come to Him and merely repeat it. For this He obscurely signified when He said, “They think that they shall be heard for their much speaking.”
“For He knows,” says He, “what things you have need of.” And if He know, one may say, what we have need of, wherefore must we pray? Not to instruct Him, but to prevail with Him; to be made intimate with Him, by continuance in supplication; to be humbled; to be reminded of your sins.
6. “After this manner, therefore, pray ye,” says He: “Our Father, which art in heaven.”
See how He straightway stirred up the hearer, and reminded him of all God's bounty in the beginning. For he who calls God Father, by him both remission of sins, and taking away of punishment, and righteousness, and sanctification, and redemption, and adoption, and inheritance, and brotherhood with the Only-Begotten, and the supply of the Spirit, are acknowledged in this single title. For one cannot call God Father, without having attained to all those blessings. Doubly, therefore, does He awaken their spirit, both by the dignity of Him who is called on, and by the greatness of the benefits which they have enjoyed. But when He says, “in Heaven,” He speaks not this as shutting up God there, but as withdrawing him who is praying from earth, and fixing him in the high places, and in the dwellings above.
He teaches, moreover, to make our prayer common, in behalf of our brethren also. For He says not, “my Father, which art in Heaven,” but, “our Father,” offering up his supplications for the body in common, and nowhere looking to his own, but everywhere to his neighbor's good. And by this He at once takes away hatred, and quells pride, and casts out envy, and brings in the mother of all good things, even charity, and exterminates the inequality of human things, and shows how far the equality reaches between the king and the poor man, if at least in those things which are greatest and most indispensable, we are all of us fellows. For what harm comes of our kindred below, when in that which is on high we are all of us knit together, and no one has anything more than another; neither the rich more than the poor, nor the master than the servant, neither the ruler than the subject, nor the king than the common soldier, nor the philosopher than the barbarian, nor the skillful than the unlearned? For to all has He given one nobility, having vouchsafed to be called the Father of all alike.
Source: Homilies on the Gospel of St. Matthew (New Advent)