Isaiah 1 speaks of rebelling against understanding, dealing corruptly and forsaking and despising the Lord. These are serious accusations growing from the root of pride. The result is personal sickness (v.5-6) and cultural desolation (v. 7).
Sin is always worse than I think; there is no little or harmless sin. And, its ramifications negatively impact me more than I think.
If verse 18 were not in chapter 1, what hope would I have? But, thank God, the promise is there. God is able to take scarlet, crimson sins and deal with them. The result of His work is purity.