It means to place your faith solely in the work of the cross and nothing that you can do. Ask yourself what is the object of my faith? Is it keeping commandments or is it loving your neighbor? The object of one's faith(most of you have never heard that term) must be in the cross. The cross is a sacrifice that paid what man could not pay no matter how hard we try. While all these things are good, self denial, bible study, prayer, fasting, being kind, helping your neighbor, and so forth. They do not work unless our faith is in the work of the cross. The RYR kept the commandments, but Jesus said if you want to be perfect take up the cross and follow me. He was speaking of His cross the only one that can save man and make him righteous. The Just shall live by faith, faith in what, bible study or the cross, keeping commandments or the cross. The object of our faith must be the cross and the cross alone and nothing we can add or do.