Psalm 51 (KJV)
51 Have mercy upon me, O God, according to thy lovingkindness: according unto the multitude of thy tender mercies blot out my transgressions.

Have mercy opon me, O God - This is the utterance of a full heart; a heart crushed and broken by the consciousness of sin. The psalmist had been made to see his great guilt; and his first act is to cry out for mercy. There is no attempt to excuse his sin, or to apologise for it; there is no effort to vindicate his conduct; there is no complaint of the righteousness of that holy law which condemned him. It was “guilt” that was before his mind; guilt only; deep and dreadful guilt. The appeal properly expresses the state of a mind that is overwhelmed at the remembrance of crime, and that comes with earnestness to God to plead for pardon. The only hope of a sinner when crushed with the consciousness of sin is the mercy of God; and the plea for that mercy will be urged in the most earnest and impassioned language that the mind can employ. “Accordingly to thy Iovingkindness.” On the meaning of the word used here, see the notes at Psalm 36:7.