The prayer of the Christian is made possible by Jesus Christ. Apart from Christ — or the covenant, more generally speaking — prayer can only be a speculation (a hope without certitude) that something or someone will care for us. In Christ, prayer is a knowledge and certainty that Someone has cared for us and will care for us.
As I’ve said in the past, the best theologians have the best things to say on prayer: Forsyth, Baillie, Barth, and von Balthasar. Here is a nice short presentation from Emil Brunner, with his characteristic focus on the prior necessity of a particular revelation (the God who saved/saves) in order to even pray:
The world often seems like a monstrously sinister machine, blind, insensible, destroying everything that man builds, fosters, loves, hopes. Why should the world concern itself about your wishes, little stupid man? What does your sigh mean in the midst of a universe where suns grow and age in billions of years? Such a thought makes prayer die upon the lips. Is there any sense in praying the roaring avalanche to spare the babe yonder in the path of its downward rush? O fate, blind, awful, senseless fate!
When we look beyond ourselves out into the world, prayer fades away. Man’s tragic lot robs one of the courage to pray. Everything appears to be senseless, disorder, chance, confusion. Who then has a mind to pray? The world can at most permit us dimly to perceive a mysterious Power; but to make us trust ourselves to this Power, calling upon it as children do their father: “Help us!” the world is unable. How then can we pray? What gives us the courage, the confidence, the assurance?
As children lost in a woods, are fearful of the sinister darkness — and then, suddenly, hearing a sound from the sombre blackness, a familiar voice, a loving, seeking, helping voice, their mother’s voice — so prayer is our reply to the voice from the Word of God in Jesus Christ which suddenly cries out to us in the mysterious, dark universe. It is the Father calling us out of the world’s darkness. He calls us, seeks us, wants to bring us to Himself. “Where are you, my child?” Our prayers mean “Here I am, Father. I was afraid until you called. Since you have spoken, I am afraid no longer. Come, I am waiting for you, take me, lead me by the hand through the dark terrifying world.”
Because they do not know this God and this revelation so many men of our time no longer pray. True prayer is possible only as an answer to God’s real revelation. True prayer, that is, prayer in which a man really believes he will be heard, is possible only when one believes in the living God. What is meant by the “living God”? The God to Whom you can pray trustfully, because He has previously revealed to you His trustworthiness. That is the living God.
(Emil Brunner, Our Faith, trans. John W. Rilling, New York: Charles Scribner’s Sons, 1954 , pp. 110-113.)