It seems such a strange request from a man who's in the throes of grief over sins that he can't deny and can't take back. I would propose to you that it was exactly the right thing for David and each of us to pray whenever we're confronted with our sins. But when you first read the word in Psalm 51, it does make you wonder, "What in the world is hyssop?"
Researching the plant won't give you much help. It produces a delicate white flower and is thought by some to have medicinal qualities. But this is one time that wikipedia.com won't help you. What you really need to know, in order to understand the grieving in David's request, is Old Testament history. David's mind goes to that original Passover, when the firstborn of Egypt were stricken dead and the houses of Israel that had blood on the door frames, were passed over. What does this have to do with David's request? Here it is. God directed the Israelites to take a branch of hyssop and dip it in blood and paint the door frames with it.
Here is David, grieved by his sin and bowed before God between the "already" and the "not yet." Already the blood of the first Passover had protected Israel from death and made their exodus to freedom and the land of promise possible. Already the Mosaic system of constant animal-blood sacrifices covered the sins of God's people. But the promised Lamb had not yet come. Not yet had his blood been spilt, once and for all, in the final moment of sacrifice that forever ended any need for further sacrifice.
So, reflecting on the past, David's words actually reach into the future. They form the ultimate backdrop to the future prayer. For embedded in this cry for cleansing
that remembers the spilt blood of deliverance (Passover) and the shed blood of forgiveness (Mosaic sacrifices), David cries for the one thing that anyone who acknowledges his sin will cry for; cleansing.
When your sin really does become ugly to you, when it produces pain in your heart and sickness in your stomach, you celebrate forgiveness, but you want something more. You want to be clean. You long to be once and for all purified from all sin whatsoever. You want your sin to be once and for all washed away. You want to be free of every dark residue of sinful thought, desire, word, or deed.
Yes, you'll love the fact that you can stand before God dirty and unafraid because of his comprehensive and freely given grace. You'll love the fact that his forgiveness of you has been full and complete. But you'll grow tired of needing and seeking forgiveness. You'll mourn the hold that sin has on you. You'll be frustrated with the way that sin seems to infect everything you do. And you'll begin to plead for what the blood of Jesus alone is able to do; wash away your sin! In this moment of need and helplessness, you'll cry, "Purge me with hyssop Lord, dip the branch of your grace into the blood of your Son and cleanse me once and for all!"
David never sang this great old hymn, but maybe he'll hear it some day and remember the tear-stained prayer that followed the visit of Nathan. Maybe someday he'll celebrate final cleansing with a chorus of the ages singing:
"What can wash away my sin?
Nothing but the blood of Jesus.
What can make me whole again?
Nothing but the blood of Jesus.
O' precious is the flow,
That makes me white as snow.
No other fount I know,
Nothing but the blood of Jesus."