Originally from here.
God’s Law serves at least three purposes, which Calvin wisely expressed. He affirmed that the Law first serves as a mirror for us. It reveals the perfect character of God, and in so doing, it exposes our sin. This might be called the schoolmaster function. The Law instructs us in our need for God’s grace. It reveals His perfection and our failure to measure up. It reveals our need for Christ.
The second use is often called the civil use of the Law. Here Calvin argued that those outside the kingdom are restrained by the revelation of the Law. It doesn’t change the heart of the unregenerate, but it can create a sense of fear. As the civil law reflects God’s revealed Law, and with it, civil sanctions, it restrains the wicked.
The third use of the Law is likely the most controversial. Calvin argued that the Law reveals to us that which is pleasing to God. That is, it tells us what to do. As we obey it we please Him. Some fear that in embracing this third use we muddy up the first use. If we argue with respect to the Law, “This you must do” are we not at least obscuring the truth that “This you cannot do”?
My fear, however, operates in the other direction. If we obscure the third use of the Law we obscure the first use of the Law. That is, if we are not called and required to follow the commands of God, our failure to do so doesn’t mean we are at enmity with God. The schoolmaster cannot tell us of our need for atonement if we have not failed to do what we are called to do. Secondly, however, without the third use of the Law we end up worse off than the heathen. We don’t know what to do. We are left without direction. If we are not called to do what the Law of God says, how will we decide what to do?
Some will say, “Let love decide.” Great answer. Trouble is, the Great Commandment, which calls us to love God and our neighbor, is that which binds up all the Law and the prophets (Matthew 22:40). Which means that “love” is not a new, indistinct, culturally conditioned law, but is instead the Law of God. We are not left with what we think love means, by abandoning the Law, but are left with what love actually means by keeping the Law.
The third use of the Law, however, has this other benefit. We could see it as the other side of the second use coin, or as an extension of the third use. The Law tells us how to have a good life. It tells us how to be blessed. It tells us how to do what we were made to do. To put it more poetically, the Law is the gateway to joy. This is less because, especially for believers, God sends thunderbolts down on us when we disobey Him, or rose petals on us when we obey. It is more because the Law is good in itself. Obedience is blessing long before obedience brings blessing. We were made for this.
David certainly needed the Law to convict him, to point him to his need for Christ. But he sang, “Oh how I love Your Law” (Psalm 119:97) for the joy that it brings. God’s Law is not a list of pleasures we are not allowed to have, a list of delights we are not allowed to touch. It is instead pleasure and delight. Having been, while yet unbelievers, restrained, having been at our conversion convicted, having been in our walk instructed, may we be in our hearts, as we will be in eternity, ever joyful.