How can I guarantee that God will hear my prayer?
[God] does indeed bow down his ear, if you do not lift up your neck; for the Lord draws near to the humble man. But from the exalted man he distances himself, unless he himself has exalted that man for his humility. God then bows down his ear towards us. For he is on high, we below; he is in a high place, we in a low yet not yet desolate place.
from St. Augustine Answers 101 Questions on Prayer by Fr. Ermatinger.
God has given man the ability to pray, the grace to turn in prayer, the command to pray, and even the very words by which to beseech Him in prayer. As God has so given, it is without a doubt that God desires to hear man’s prayers and further to give to man the good things that he prays for. Prayer is not a calling out into the void, pleading to a god who may or may not be listening or even care. God gives and commands that He might grant that which He asks for us to pray for. It is not just that the hearing of prayer is a promise that God keeps, but that the conditions that bring about God promising are, in their own turn, gifts of grace created so that God might promise and then fulfill. This is the depths of God’s love and desire to hear man’s prayers: the conditions of life that the individual man finds himself in are not absent of God, but are precisely so that man might turn in prayer to Him.
Does God hear the prayers of sinners? Doubting so comes easily and it comes from a pride that, God being in a high place and man in a low place, exalts the low place of man and diminishes the high place of God by saying that God is not strong enough to come down, that man and his sins are too much for God to bear. Man cannot make his place a desolate place where God cannot enter. God especially hears the prayers of sinners, for this is why He created, commanded, and instructed prayer; that He might show forth that He is indeed God by granting His mercy to those who were undeserving of it. God surely hears those who do His will, who remain with Him, and abide with Him for hearing the prayers of such are the merits for abiding in Him; the crown by which He crowns His own gifts. But for those who are unmeriting of His goodness, He instead offers the path of mercy, repaying the evil that was done by man by the goodness of His charity.
What is needed, on the part of man, is humility and obedience. First obedience in turning in prayer and second humility in seeking only the good things that God wishes to give. Man has no demands upon God; not as a creature, especially not as a sinner. It is the haughty man who lifts his eyes up to God and, filled with pride, demands that God give him that which he has no right to and has lost through his sins. God hears not the prayers of the puffed-up man, who is lost in his vainglory.
God hears the prayers of the man who cries, “Mercy!” in humility. It is the faith’s acknowledgment of man’s place and dependency upon God, hope in God’s promise of salvation and good things in spite of man’s transgressions, and desire for God’s charity – not the charity that man might demand but rather the charity that God chooses; for it is humility that teaches man that whatever God so chooses to give in His mercy is both more and better than what man could dream up in his pride.