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.
Source: Homilies on the Gospel of St. Matthew (New Advent)