"God, be merciful to me, a sinner."
In Russian Orthodox circles, a "staretz" refers to a much-experienced spiritual advisor. One day it was reported to a certain staretz that a man had experienced seeing angels. Such a vision, it was thought, must mark great spiritual progress! But the staretz answered very wisely, "This is not surprising, that he sees angels, but I would marvel at a person who saw his own sins."
In Luke 18, the story of the Pharisee and the tax collector is called a parable, but it should also be viewed as a miracle. For in it, a man truly sees his own sins.
"But the tax collector, standing far off, would not even lift up his eyes to heaven, but beat his breast, saying, 'God, be merciful to me, a sinner'" (Luke 18:13).
This tax collector, normally known for taking people's money, is here giving the world a gift: the perfect (seven-word) prayer. "God, be merciful to me, a sinner."
This prayer goes to great spiritual depths and heights. It accomplishes two things. First, it keeps you from becoming proud, by remembering that you are a sinner. In Greek, it does not say "a sinner," but "the sinner." Think of yourself as the only sinner. You will never look down on another.
Second, the prayer keeps you from despairing, by remembering that God is merciful. The words "God, be merciful" are in fact a promise that God is merciful and forgiving. It is a prayer of faith and trust in God's mercies.
It seems clear that the tax collector prayed this prayer several times. You too pray it several times today - and often.