The first chapter of Isaiah sets the scene of the book of Isaiah. The people are accused of having a lack of knowledge of God, being lawless and worshipping superficially only. God reminds the people in chapter 1 that what is in the heart and head is what is important, rather than their outward signs of worship and sacrifice.
Isaiah 1 contains the beautiful words of verse 18 where God, calling on his people to return to him, promises that he will remove their sin.