The giving of the law at Mount Sinai was the climax of a series of events that began at Passover, the moment and the means of the Israelites' redemption. At Passover they killed a lamb and put the blood on their doorposts. When the death angel passed through to slay the firstborn, those who had blood on the doorposts were spared. God was saving, redeeming, buying back His people.
Mount Sinai adds the other half of the equation. Though redemption through the blood of a lamb (Christ) freed them from sin's dominion and death, the giving of the law at Mount Sinai shows that freeing them is not all that God had in mind. Israel came to Mount Sinai after being redeemed, heard the law, and assented to keep it. God gave the law to show the pattern of life, the principles of righteousness, for the redeemed.
On one side of the coin is grace and on the other is law and obedience. They are harmonious; they cannot be separated. They are both vital parts of the process of sanctification leading to salvation. Grace is given upon repentance from sin, but after repentance, what is a Christian to do with his life? Obedience to God and living a life of holiness become his first priorities, and these work to produce character in the image of God (II Corinthians 3:18).
Amos 5:25 reconfirms that the sacrifice, offering, and shedding of blood is a foundational necessity for a relationship with God. "Did you offer Me sacrifices and offerings in the wilderness forty years, O house of Israel?" The answer is, "Yes." The people were sacrificing, but is that all that they did? He implies that though they were sacrificing, something was missing—obedience to the law.
God told Israel that He would dwell in the Tabernacle, specifically the Holy of Holies, the symbolism of which we need to understand. The most important piece of furniture inside the Holy of Holies was the Mercy Seat, a wooden chest overlaid with gold. Its lid functioned as the seat. Inside the chest, under the seat, were stored the two tablets of stone, symbolizing God sitting on His law, the basis of His judgment.
When one sins, he begins to separate himself from fellowship with God (Isaiah 59:1-2). He is no longer permitted, as it were, to come into the Holy of Holies. What means did God provide to heal the broken relationship, to restore the fellowship?
One might think that the giving of a sin offering would appease God, and He would forgive the sin. However, Hebrews 10:4 is very clear: "For it is not possible that the blood of bulls and goats could take away sins." Then why did God have the Israelites make these sacrifices? "But in those sacrifices there is a reminder of sins every year" (verse 3). As Amos does not mention the sin offering in Amos 5:22, it seems that Israel did not even make the attempt to be reminded of sin.
So how was fellowship restored? On the Day of Atonement, once a year, the high priest entered the Holy of Holies to sprinkle the Mercy Seat with blood. God's intent in this ritual was to show people that their transgressions of His law were covered by the blood. The redeemed were again in fellowship with God.
The blood and the law are essential parts for maintaining the correct relationship with God. The law is permanent and codifies the nature of God in precepts to help us understand Him clearly. Obedience to His law is a perpetual requirement, with blood available to cover any transgression of it.