Have mercy upon me, O God, according to Thy lovingkindness: according unto the multitude of Thy tender mercies blot out my transgressions…Purge me with hyssop, and I shall be clean: wash me, and I shall be whiter than snow. – Psalm 51:1,7

Do not appeal to justice, sinner. That is against you; appeal to mercy. “Have mercy upon me, O God, according to Thy lovingkindness!” This prayer he brings before God is prayer tipped with a hope in the mercy of God. Go, sinner, and plead with God and fight your sins with hope in His mercy. When he had done that, he then turns to confession: “I acknowledge my transgression, and my sin is ever before me.” There is no weapon to drive away guilty fears like making a clean breast of your sins. Tell your Father whom you have offended; do not plead any extenuations or mitigations. Confess that you deserve His wrath. Put yourself before the throne of God’s clemency. Confess that if it were turned to a throne of vengeance, you deserve it well. Prayers, tears, pleas for mercy, and full confession-these are weapons to conquer with.

“Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow.” Hyssop was a little bunch, a brush, used to dip into the blood-a basin full of blood, and then with this brush of hyssop the priest sprinkled the guilty man, the unclean man, and he was counted clean. The master argument in this verse is blood. Oh! how this destroys our sins, how this scatters all our doubts and fears-the almighty weapon of the cross, the divine weapon of the atonement. Let sins come on, and let them be more than the hairs of my head, loftier than mountains and deeper than the unfathomed ocean, let them come on-God’s flaming wrath behind them, hell itself coming to devour me; yet if I can but take the cross and hold it up before me, if I can plead the precious blood I shall be safe, for I shall be saved and proved a conqueror, notwithstanding all. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0972.cfm