1. To violate a religious, ethical, or moral law or standards; or to do something that is wrong according to religious rules: Maude Williams was sinning against the doctrines of just about every religious organization with her securities fraud, money laundering, and perjury.
2. To commit offenses or violations that are regarded as being shameful, deplorable, or utterly wrong: Those who have not murdered or committed adultery have sinned when they don't tell the truth or when they worship false idols; such as, wealth or human powers as being more important than God.

Very often, people are defensive of their sins because they have not been able to stop sinning with their own strength; for example, when they can't stop smoking, they rationalize about its presence in their lives by making the humiliating confession that they "can't conquer it"; so, they present arguments that there is no one who can be perfect.