The fourfold purpose of the law

Calvin taught on the threefold use of the law:

1. The law teaches us.
It teaches us about ourselves, by acting as a mirror to show us our nature.
Romans 3:20b "For by the law [is] the knowledge of sin"
Romans 4:15 "For the law brings wrath, but where there is no law there is no transgression"
Romans 7:7-11 "What then shall we say? That the law is sin? By no means! Yet if it had not been for the law, I would not have known sin. For I would not have known what it is to covet if the law had not said, “You shall not covet.” But sin, seizing an opportunity through the commandment, produced in me all kinds of covetousness. For apart from the law, sin lies dead. I was once alive apart from the law, but when the commandment came, sin came alive and I died. The very commandment that promised life proved to be death to me. For sin, seizing an opportunity through the commandment, deceived me and through it killed me."
It acts in fact, like an instructor. This is what Paul says in Galatians 3:19a- "What purpose then [does] the law [serve]?" He answers it in v23-24 "But before faith came, we were kept under guard by the law, kept for the faith which would afterward be revealed. Therefore the law was our tutor [to bring us] to Christ, that we might be justified by faith."

The law also teaches us about God. Being the one who gave the law, the law comes from Him. The law is good, and pure, and right, "Therefore the law [is] holy, and the commandment holy and just and good." Romans 7:12. By studying the law we can learn about God's character. He gave us the law from His character, and by learning the law, we  learn about Him.

2. The law restrains evil.
Romans 13:3 "For rulers are not a terror to good works, but to evil. Do you want to be unafraid of the authority? Do what is good, and you will have praise from the same."
Deuteronomy 13:10-11 "And you shall stone him with stones until he dies, because he sought to entice you away from the LORD your God, who brought you out of the land of Egypt, from the house of bondage. "So all Israel shall hear and fear, and not again do such wickedness as this among you"

When faithfully practiced the preserved society, making it harmonious to live in. We are blessed to have rules and order, good conduct, and a well oiled machinery to support us. It makes life pleasant not to be stabbed, when trying to get groceries, after all.

3. The law teaches Christians how to please God.
1 John 2:4 "He who says, "I know Him," and does not keep His commandments, is a liar, and the truth is not in him."
1 John 3:22 "And whatever we ask we receive from Him, because we keep His commandments and do those things that are pleasing in His sight."
1 John 5:2 "By this we know that we love the children of God, when we love God and keep His commandments."
1 John 5:3 "For this is the love of God, that we keep His commandments. And His commandments are not burdensome."
John 15:10 "If you keep My commandments, you will abide in My love, just as I have kept My Father's commandments and abide in His love."

The law exists so we can obey God, and by submitting to Him and following His rules. We might like to say that we love God, or we love His church, or we love His doctrines, but if we do not obey His commandments, we have not actually loved God. If there is a Christian who reads the Bible and says "Well okay, but I don't want to do any of that." that person is not a Christian.

Now all this is well tread and nothing new. But I think, as I consider the sermon on the mount, that the law has another purpose - it's to reveal Christ.
This is the same as John's purpose. If you remember, his job was simply to baptize everyone, so that when the messiah came John would baptize him and something would happen to make him known, to mark him out as special. That's what happened, and in that moment the purpose of John - to reveal Christ - was fulfilled. John 1:31 "I did not know Him; but that He should be revealed to Israel, therefore I came baptizing with water."
The law was not just pointing people on to Christ indirectly because it shows them they are too small to keep it, it is itself testifying to Christ.
The law was holding back, waiting, biding time, uniformly condemning everyone so that the person who was not not condemned would stand out like a sore thumb. The law itself magnifies and lifts Christ up above everyone else in the same way John the Baptist did.
And that I think is the fourth purpose of the law.