When Big Brother and Little Brother Fight

Religion teaches duty and laws and judgment. It teaches about a god who demands perfection from us Christianity also teaches about duty, law and judgment, but instead of looking at our attempts to obey, it teaches about the God who gives perfection found through Jesus alone.
Like so many sibling squabbles, faith vs duty or works has been fought over and debated for centuries. Even though God's Word through the Apostles encourages us with the fact that "by faith we are saved, not by works." Why do we have such a hard time seeing that living out our lives (works) should be done in faith? Acts 15:1-21