The very question “Does prayer work?” puts us in the wrong frame of mind from the outset. “Work”: as if it were magic, or a machine—something that functions automatically. Prayer is either a sheer illusion or a personal contact between embryonic, incomplete persons (ourselves) and the utterly concrete Person. Prayer in the sense of petition, asking for things, is a small part of it; confession and penitence are its threshold, adoration its sanctuary, the presence and vision and enjoyment of God its bread and wine. In it God shows Himself to us. That He answers prayers is a corollary—not necessarily the most important one—from that revelation. What He does is learned from what He is.
Petitionary prayer is, nonetheless, both allowed and commanded to us: “Give us our daily bread.” And no doubt it raises a theoretical problem. Can we believe that God ever really modifies His action in response to the suggestions of men? For infinite wisdom does not need telling what is best, and infinite goodness needs no urging to do it. But neither does God need any of those things that are done by finite agents, whether living or inanimate. He could, if He chose, repair our bodies miraculously without food; or give us food without the aid of farmers, bakers, and butchers; or knowledge without the aid of learned men; or convert the heathen without missionaries. Instead, He allows soils and weather and animals and the muscles, minds, and wills of men to co-operate in the execution of His will.
“God,” said Pascal, “instituted prayer in order to lend to His creatures the dignity of causality.” But not only prayer; whenever we act at all He lends us that dignity. It is not really stranger, nor less strange, that my prayers should affect the course of events than that my other actions should do so. They have not advised or changed God’s mind—that is, His over-all purpose. But that purpose will be realized in different ways according to the actions, including the prayers, of His creatures.
– C.S. Lewis