I believe that justification by faith is the foundational truth of Christianity. You can't know true rest and peace until you are convinced you can never be made right in God's eyes by your own works of righteousness.
If you don't understand the perfect righteousness of Christ that is yours by faith, you'll lead a life of toil and sweat. You'll spend your days trying to please God through some kind of legalistic, hopeless attempt to establish your own righteousness. But the truth is, you'll never have any righteousness to bring to the Lord!