“‘Have mercy upon me, O God.’ (Psalm 51:1) He appeals at once to the mercy of God, even before he mentions his sin. The sight of mercy is good for eyes that are sore with penitential weeping. Pardon of sin must ever be an act of pure mercy and therefore to that attribute the awakened sinner flies.”
By Charles Spurgeon, The Treasury of David Vol. 1 (Peabody, MA; Hendrickson Publishers, 1988) p. 402. Commentary on Psalm 51:1/ theoldguys.org/category/gods-mercy.