Romans 3
"For we maintain that a man is justified by faith apart from works of the Law" (v28). After saying, "he is a Jew who is one inwardly" (2:29), Paul's hearers were wondering if the Law was useful at all. Clearly, "by the works of the Law no flesh will be justified in His sight" (v20). However, through the Law we understand sin, and the Law displays God's holiness and points us toward Him (v1-8).
If we think "doing good" justifies us, then our faith is in ourselves, not God. God Himself is always the ultimate goal.