The passage may be read as the voice of faith as well as a prayer, and so it runs -- "Thou wilt purge me with hyssop, and I shall be clean." Foul as I am, there is such power in the divine propitiation, that my sin shall vanish quite away. Like the leper upon whom the priest has performed the cleansing rites, I shall again be admitted into the assembly of thy people and allowed to share in the privileges of the true Israel; while in thy sight also, through Jesus my Lord, I shall be accepted. Wash me. Let it not merely be in type that I am clean, but by a real spiritual purification, which shall remove the pollution of my nature. Let the sanctifying as well as the pardoning process be perfected in me. Save me from the evils which my sin has created and nourished in me. And I shall be whiter than snow. None but thyself can whiten me, but thou canst in grace outdo nature itself in its purest state. Snow soon gathers smoke and dust, it melts and disappears; thou canst give me an enduring purity. Though snow is white below as well as on the outer surface, thou canst work the like inward purity in me, and make me so clean that only an hyperbole can set forth my immaculate condition. Lord, do this; my faith believes thou wilt, and well she knows thou canst.
Scarcely does Holy Scripture contain a verse more full of faith than this. Considering the nature of the sin, and the deep sense the psalmist had of it, it is a glorious faith to be able to see in the blood sufficient, nay, all sufficient merit entirely to purge it away. Considering also the deep natural inbred corruption which David saw and experienced within, it is a miracle of faith that he could rejoice in the hope of perfect purity in his inward parts. Yet, be it added, the faith is no more than the word warrants, than the blood of atonement encourages, than the promise of God deserves. O that some reader may take heart, even now while smarting under sin, to do the Lord the honour to rely thus confidently on the finished sacrifice of Calvary and the infinite mercy there revealed."
Quoted from 'The Treasury of David, Classic Reflections on the Wisdom of the Psalms' by C H Spurgeon, 3 vol set, £39.95