Thomas Aquinas, Compendium of Theology

In the order of divine providence, each being has assigned to it a way of reaching its end in keeping with its nature. To men, too, is appointed a suitable way, that befits the conditions of human nature, of obtaining what they hope for from God. Human nature inclines us to have recourse to petition for the purpose of obtaining from another, especially from a person of higher rank, what we hope to receive from him. And so prayer is recommended to men, that by it they may obtain from God what they hope to secure from Him.

But the reason why prayer is necessary for obtaining something from a man is not the same as the reason for its necessity when there is question of obtaining a favor from God. Prayer is addressed to man, first, to lay bare the desire and the need of the petitioner, and secondly, to incline the mind of him to whom the prayer is addressed to grant the petition. These purposes have no place in the prayer that is sent up to God. When we pray we do not intend to manifest our needs or desires to God, for He knows all things. The Psalmist says to God: "Lord, all my desire is before Thee" (Psalm 37:10); and in the Gospel we are told: "Your Father knoweth that you have need of all these things" (Matt. 6:32). Again, the will of God is not influenced by human words to will what He had previously not willed. For, as we read in Numbers 23:19, "God is not a man, that He should lie, nor as the son of man, that He should be changed"; nor is God moved to repentance, as we are assured in I Kings 15:29. Prayer, then, for obtaining something from God is necessary for man on account of the very one who prays, that he may reflect on his shortcomings and may turn his mind to desiring fervently and piously what he hopes to gain by his petition. In this way he is rendered fit to receive the favor.

Yet a further difference between the prayer offered to God and that addressed to man is to be marked. Prayer addressed to a man presupposes a certain intimacy that may afford the petitioner an opportunity to present his request. But when we pray to God, the very prayer we send forth makes us intimate with Him, inasmuch as our soul is raised up to God and converses with Him in spiritual affection, and adores Him in spirit and truth. The familiar affection thus experienced in prayer begets an inducement in the petitioner to pray again with yet greater confidence. And so we read in Psalm 16:6: "I have cried to Thee," that is, in trusting prayer, "for Thou, O God, hast heard me"; as though, after being admitted to intimacy in the first prayer, the Psalmist cries out with all the greater confidence in the second.

For this reason, in prayer to God, perseverance or repetition of our supplication is not unseemly, but is regarded as acceptable to God. Indeed, "we ought always to pray and not to faint," as we learn from Luke 18:1. Our Lord, too, invites us to pray, for He said: "Ask, and it shall be given you . . . knock, and it shall be opened to you" (Matt. 7:7). But in prayer addressed to man, persistence in begging becomes irritating.