The (51st) 50th psalm is the famous psalm of repentance. It is a collection of 21 verses. Each verse is like a diamond. The first one starts: “Have mercy upon me, O God, according to your great mercy and according unto the multitude of your compassions blot out my transgressions”.
  The person who is asking for help here is the most glorious King of Israel. But in the first verse he appears as a poor beggar. People are indeed unhappy and live in a void in spite the astonishing progress in the technical and scientific fields. They suffer and groan even if they live in a wealth. The machines they invented to make life easier and happier, made them their slaves. This kind of civilization has not liberated people from passions and evils but added an oppressive weight. They feel the need of deliverance from the internal slavery of the passions and from the external slavery of the modern world.
  The poet asks God’s mercy. But a distinction is made between small and great mercy. Small mercy concerns all the earthly goods which the omnipotent God gives us. For example, the rain which falls and moistens the fields and makes the earth bloom. The air which the whole planet and all mankind breathes. The rays of the sun which illuminate and warm the earth. These three blessings and many more are given freely to all people without exception. As Christ says, the heavenly Father: “makes his sun to rise on the evil and on the good, and sends rain on the just and on the unjust.” (Mt 5:45) Therefore, God is merciful to all of us, even to those who receive all the blessings of divine creation but do not feel the need to express their gratitude to God.
  My God! David, the repentant sinner, cries out. I feel my guilt. I see the deep corruption of my heart. My parents are sinful because they are descendants of Adam and Eve. From the first couple originates the inclination and tendency towards sin. I do not want to sin. But my will is very weak. I fall into sin. However, I am not senseless and indifferent. A voice, like a spark, inside me does not stop censuring me as an offender of the divine law. I have nothing to offer you but a pitiful and unclean heart. O Lord, I ask for your great mercy.
 staurosi This great mercy is the forgiveness of sins. It derives only through Christ’s sacrifice on the cross. Christ’s Blood is the endless ocean of God’s mercy and compassion. He washes, cleans and regenerates the sinner. “Have courage, my son. Your sins are forgiven” (Mt 9:2). This is the answer of heaven to the sinner, who by having a broken and repentant heart falls down and worships the Redeemer of the world, saying this prayer of repentance: “Have mercy on me, O God ...”.