Romans 3:20

Romans 3:20. Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin,

All the law can do is to show us our sin. The law is a mirror, and looking in it you can see your spots; but you cannot wash in a looking-glass. If you want to be cleansed from your stains, you must go somewhere else. The object of the law of God is not to cleanse us, but to show us how much cleansing we need; to reveal our disease, not to find a remedy for it.