Hester is spared execution only because the “Puritans of Boston” decided it would benefit the community to transform her into a “living sermon against sin”.
A sin may mean differently depending on your religion, point of views, or how severe the committed act is. For example, people lie all the time it can be a small, innocent lie or it can be a huge lie that can harm someone; People may think that is a sin but others don’t. I believe a lot of people never notice when they sin and if they do they don’t care. People may think they are not sinner and that they are perfect but everyone is a sinner. My definition o f a sin is any act that harms God or anything against the church.
In the Scarlet Letter, everyone is a sinner not only Hester because she committed adultery she changes throughout the book and that makes her a better person. All of the other characters are sinners as well. For example, I think Dimmesdale might be the worst sinners.