There are several offenses against common sense in your question...
" Among the ancients there was a threefold error concerning prayer. Some held that human affairs are not ruled by Divine providence; whence it would follow that it is useless to pray and to worship God at all: of these it is written (Malachi 3:14): "You have said: He laboreth in vain that serveth God." Another opinion held that all things, even in human affairs, happen of necessity, whether by reason of the unchangeableness of Divine providence, or through the compelling influence of the stars, or on account of the connection of causes: and this opinion also excluded the utility of prayer. There was a third opinion of those who held that human affairs are indeed ruled by Divine providence, and that they do not happen of necessity; yet they deemed the disposition of Divine providence to be changeable, and that it is changed by prayers and other things pertaining to the worship of God. All these opinions were disproved in I, 19, 7,8; I, 22, 2,4; I, 115, 6; I, 116. Wherefore it behooves us so to account for the utility of prayer as neither to impose necessity on human affairs subject to Divine providence, nor to imply changeableness on the part of the Divine disposition.
In order to throw light on this question we must consider that Divine providence disposes not only what effects shall take place, but also from what causes and in what order these effects shall proceed. Now among other causes human acts are the causes of certain effects. Wherefore it must be that men do certain actions. not that thereby they may change the Divine disposition, but that by those actions they may achieve certain effects according to the order of the Divine disposition: and the same is to be said of natural causes. And so is it with regard to prayer. For we pray not that we may change the Divine disposition, but that we may impetrate that which God has disposed to be fulfilled by our prayers in other words "that by asking, men may deserve to receive what Almighty God from eternity has disposed to give," as Gregory says (Dial. i, 8) "