“The ultimate purpose of God’s law is not to give us a staircase to climb to heaven. Scripture is clear that the law itself can never make us righteous or earn favor with God. The law, rather, exposes our sin and shows us our need for grace. Human beings seem hard-wired to trust in their own performance as the basis of their acceptance with God. We may do this by trying to keep the Ten Commandments or live by the Sermon on the Mount. But while it is true that God’s law reflects his divine character and sets the bar by which we are judged, Scripture teaches that it is impossible to achieve a right standing with God through obedience (Gal. 3:21). This simply isn’t the purpose of God’s law. The attempt to gain God’s acceptance through law-keeping or performance is legalism. But the law was given to show us that we cannot satisfy God by what we do. In effect, the law gives us something tangible to bang our stubborn heads against until we throw up our hands and say, “I can’t do this. I can’t satisfy God’s requirements of me. This is impossible! I need someone else to do it. I despair of my own strength. Help me.” The law, therefore, is like an X-Ray machine: useful for diagnosis, but not for cure; able to reveal a fracture, but not to reset a bone. The law shows us that we fall short, but cannot change our status in any way. It makes us conscious of sin, but has no power to rescue from sin’s curse and control. This is the significance of Paul’s statement in Romans 3:
‘We know that whatever the law says it speaks to those who are under the law, so that every mouth may be stopped, and the whole world may be held accountable to God. For by works of the law no human being will be justified in his sight, since through the law comes knowledge of sin.’ —Romans 3:19–20

Exerted from Christ Formed in You:  The Power of The Gospel for Personal Change by Brian Hedges (Kindle edition location 950-980)