Those who have repented and are striving to obey God in faith are the ones under grace. We are not justified by the law. Because the law only gives knowledge of sin. The purpose of the law is not to forgive or justify or wash away our sins. Only the blood of Jesus Christ is able to do that.
Sin is the transgression of the law, or lawlessness.
That is what sin is. The purpose of the law is to define sin. To reveal it to us so that we can repent of transgressing it. By the law comes knowledge of sin. It is the spiritual mirror that allows us to see our faults and shortcomings, But be ye doers of the word, and not hearers only, deceiving your own selves.