Why did God give the Mosaic Law? What is the purpose of the Mosaic Law?

TL;DR:

God gave the Israelites the Mosaic Law to reveal how He called them to live. The Mosaic Law was for the people’s own good, to reveal Himself to them, to set them apart so He could reveal Himself to others, and to reveal humanity’s need for a savior.

what does the bible say?

The Mosaic Law refers to the laws God gave Moses on Mount Sinai after releasing the Israelites from slavery in Egypt. They include the Ten Commandments, ordinances for living in society, and regulations for worship (requirements for priests, sacrifices, feasts, and the temple). The Bible gives at least four reasons God gave the Mosaic Law to His people: for their own good, to reveal Himself to them, to set them apart in order to reveal Himself to others, and to reveal humanity's need for a savior.

from the old testament

When the Law was presented to the Israelites in Deuteronomy 10:13, God told them, "to keep the commandments and statutes of the LORD, which I am commanding you today for your good."
God intended for the Mosaic Law to be a blessing for His people. We easily see this in laws like "You shall not murder" (Exodus 20:13), "You shall not steal" (Exodus 20:15), and "you shall love your neighbor as yourself" (Leviticus 19:18). Societies where these laws are followed are safer and more prosperous than societies without similar requirements.
Deuteronomy 5:29 and 33 further reveal the heart of God in prescribing the Mosaic Law. God says, "Oh that they had such a heart as this always, to fear me and to keep all my commandments, that it might go well with them and with their descendants forever!" and "You shall walk in all the way that the LORD your God has commanded you, that you may live, and that it may go well with you, and that you may live long in the land that you shall possess."
Hundreds of years later, the psalmist, while reflecting on God's Law, wrote that he loved it and that God's words were sweeter than honey because the Law gave him wisdom and understanding (Psalm 119:97–104). This psalmist certainly experienced the laws as the blessing God intended them to be.
God also states that the Law was to teach the people that He is holy and to be feared (respected). In Leviticus 19:2, God says, "Speak to all the congregation of the people of Israel and say to them, You shall be holy, for I the LORD your God am holy."
Deuteronomy 31:12–13 show us how the Law is meant to continue revealing God's holy and awesome character to future generations: "…be careful to do all the words of this law, and that their children, who have not known it, may hear and learn to fear the LORD your God, as long as you live in the land that you are going over the Jordan to possess." God’s holiness is revealed in many of the laws prescribing worship and the way the people and priests were required to approach the Lord's presence.
God gave the Mosaic Law to the Israelites to set them apart as different from other nations. He told them, "Keep my statutes and do them; I am the LORD who sanctifies you" (Leviticus 20:8). To sanctify means to set apart for a holy purpose.
Deuteronomy 28:1, 9–10 help us understand what purpose God had for setting apart the Israelites with the Law: "And if you faithfully obey the voice of the LORD your God, being careful to do all his commandments that I command you today, the LORD your God will set you high above all the nations of the earth. … The LORD will establish you as a people holy to himself, as he has sworn to you, if you keep the commandments of the LORD your God and walk in his ways. And all the peoples of the earth shall see that you are called by the name of the LORD, and they shall be afraid of you." God intended that people in other nations would know Him better by seeing the Israelites and the laws they followed.
In Deuteronomy 4:6–8, Moses says, "Keep them [God's statues and rules] and do them, for that will be your wisdom and your understanding in the sight of the peoples, who, when they hear all these statutes, will say, 'Surely this great nation is a wise and understanding people.' For what great nation is there that has a god so near to it as the LORD our God is to us, whenever we call upon him? And what great nation is there, that has statutes and rules so righteous as all this law that I set before you today?" God wanted other nations to notice His special interactions with the Israelites. In fact, He said, "Behold, I am making a covenant. Before all your people I will do marvels, such as have not been created in all the earth or in any nation. And all the people among whom you are shall see the work of the LORD, for it is an awesome thing that I will do with you" (Exodus 34:10). The Mosaic Law was supposed to reveal God to others by setting the Israelites apart.

from the new testament

The final, and perhaps most important, purpose the Bible gives for the Mosaic Law is that it reveals humanity's need for a savior. The Law sets the standard for holiness and teaches what qualifies as sin, faults, and shortcomings. Paul says "… through the law comes knowledge of sin" (Romans 3:20), "… if it had not been for the law, I would not have known sin" (Romans 7:7). The Law reveals the myriad and plethora of ways humans fail to live up to God's standard.

implications for today

The awareness of our own sin could leave us defeated and hopeless, but God intervened. Romans 8:3–4 tell us, "For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh, in order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit." God's Son, Jesus, lived a sinless life, fulfilling all of the Law, and He offers that righteousness to any who repent and believe. In his letter to the Galatians, Paul explains, "So the law was put in charge to lead us to Christ that we might be justified by faith" (Galatians 3:24, [NIV 1984]). Our need for God's saving grace through Jesus is only clear to us if we have a standard in place, like the Mosaic Law.

understand

The Mosaic Law called the Israelites to live in a way that reflected God’s character.
The Mosaic Law exposed human sinfulness.
The Mosaic Law reflected God’s goodness and holiness and was for His people's good.

reflect

How does understanding the purpose of the Mosaic Law change the way you view God’s expectations for our lives today?
How do you see God’s laws as a blessing in guiding us toward a life that reflects God’s character?
How has the awareness of the Mosaic Law's role in revealing human sinfulness affected your relationship with God and understanding of grace?

engage

What do we learn about God from the way He set the Israelites apart, including by giving them the Mosaic Law?
How does the Mosaic Law reveal God’s holiness and goodness to other nations, and how is it similar to how God calls us to live today?
How can we understand the purpose of the Mosaic Law while recognizing the ultimate fulfillment in Christ?