A weak understanding of what the Bible says about sin is tied to a weak understanding of what the Bible says is achieved by the cross.
The more clearly we see sins horror, the more we shall treasure the cross.
Sin corrupts even our good deeds. We injure our shoulder trying to pat ourselves on the back.
Sin defies God, utterly corrupts each individual, corrodes all social relationships, and issues in death.
It is a cheap zeal that reserves its passions to combat only the sins and temptations of others.