33 Saint Augustine, in a homily on the First Letter of John , describes very beautifully the intimate relationship between prayer and hope. He defines prayer as an exercise of desire. Man was created for greatness—for God himself; he was created to be filled by God. But his heart is too small for the greatness to which it is destined. It must be stretched. “By delaying [his gift], God strengthens our desire; through desire he enlarges our soul and by expanding it he increases its capacity [for receiving him]”. Augustine refers to Saint Paul, who speaks of himself as straining forward to the things that are to come (cf. Phil 3:13). He then uses a very beautiful image to describe this process of enlargement and preparation of the human heart. “Suppose that God wishes to fill you with honey [a symbol of God's tenderness and goodness]; but if you are full of vinegar, where will you put the honey?” The vessel, that is your heart, must first be enlarged and then cleansed, freed from the vinegar and its taste. This requires hard work and is painful, but in this way alone do we become suited to that for which we are destined. Even if Augustine speaks directly only of our capacity for God, it is nevertheless clear that through this effort by which we are freed from vinegar and the taste of vinegar, not only are we made free for God, but we also become open to others. It is only by becoming children of God, that we can be with our common Father. To pray is not to step outside history and withdraw to our own private corner of happiness. When we pray properly we undergo a process of inner purification which opens us up to God and thus to our fellow human beings as well. In prayer we must learn what we can truly ask of God—what is worthy of God. We must learn that we cannot pray against others. We must learn that we cannot ask for the superficial and comfortable things that we desire at this moment—that meagre, misplaced hope that leads us away from God. We must learn to purify our desires and our hopes. We must free ourselves from the hidden lies with which we deceive ourselves. God sees through them, and when we come before God, we too are forced to recognize them. “But who can discern his errors? Clear me from hidden faults” prays the Psalmist ( Ps 19:12 [18:13]). Failure to recognize my guilt, the illusion of my innocence, does not justify me and does not save me, because I am culpable for the numbness of my conscience and my incapacity to recognize the evil in me for what it is. If God does not exist, perhaps I have to seek refuge in these lies, because there is no one who can forgive me; no one who is the true criterion. Yet my encounter with God awakens my conscience in such a way that it no longer aims at self-justification, and is no longer a mere reflection of me and those of my contemporaries who shape my thinking, but it becomes a capacity for listening to the Good itself.
Source: Spe Salvi (Vatican.va)