This is the promise of the Lord about sin. Human as we are, we really sin (Rom. 3:23). But, He will cleanse us from all of our sins. As white as now, He will make us pure again.
He is faithful and just to forgive us our sins and purify us from all unrighteousness (1 John 1:9). This verse was written because of the terrible sins that Israelites did. God was very disappointed with them. But because of His great love to us, He is still faithful to cleanse as as white as snow.