Psalm Six is one of the seven penitential psalms, and the first. The Navarre Commentary states of this Psalm that “The peaceful nights referred to in the previous psalms are sometimes disturbed by sorrowful feelings. This psalms provides a pattern for prayer in such circumstances, particularly when a person is very conscious of his own sinfulness.”
Saint John Fisher comments at length on this Psalm of the need for us to recognize our sinfulness and do penance. God will not punish twice the offenses of the sinner. If we recognize our sinfulness, ask forgiveness, and do penance out of the love of God, we will be forgiven. In the passage from Luke, as well as a similar passage from Matthew (from the Sermon on the Mount), many claim “Lord, we ate and drank in your company.” But it is not merely those that know of Christ that will be saved. Jesus says that God could raise of children of Abraham from the rocks. Jesus elsewhere asks “who are my brothers and sisters and my mother?” It won’t be blood relation or acquaintance that is our key to Heaven. It will be, rather, our humility, our rejection of pride, and our recognition of our own sinfulness and therefore helplessness before God, and a loving turn to His mercy.
Verse 9 is referenced in the passages from Luke and Matthew, but the following verse of the psalm gives us hope: “The Lord has heard my supplication; the Lord accepts my prayer.” So it won’t be the Pharisee that thanked God he wasn’t “like this tax collector” but rather, the sinner that beats his breast and asks forgiveness that will “go away justified.”