Today is the Sunday of the Publican and Pharisee, the start of the Triodion.
WHY didn’t the publican choose some majestic and moving psalm by which to pour out his heart before God, but instead had recourse to such a brief prayer?
Why did he repeat only it during the entire service?
Our answer is according to that of the holy Fathers.
When true repentance begins to shudder in the soul, when humility and contrition of spirit arises there because one’s eyes have been opened to the soul’s sinfulness, then loquacity becomes unbearable, impossible.
Concentrating within itself, turning all its attention upon its disastrous condition, the soul begins to call out to God through some form of short, concise prayer. […]
HIS eyes were directed toward the ground; and beating upon his heart with repentance from deep within his heart, he pronounced with his whole soul the prayer united with his confession: God be merciful to me, a sinner.
St Ignatius Brianchaninov (1807-1867). Translated by Nun Cornelia (Rees)
The Prayer of the Heart
LORD Jesus Christ, Son of God, have mercy on me, a sinner.