Salvation cannot be obtained by simply repenting and deciding to obey God's laws. Merely keeping the law will not justify anyone. Being justified means having one's sins forgiven and coming into a right relationship with God.
Here Paul explains that God has provided a means whereby we may receive forgiveness of sins and be accounted righteous in His sight. It is separate and distinct from obedience to the law. This forgiveness comes by having faith in the sacrifice of Jesus Christ!
Flee from these vain arguments and attitudes! Press on for righteousness! The Greek literally reads, "agonize the good agony" through the struggles involved in a Christian life (verse 12).