Beginning with Adam, we have a familial priesthood which is meant to be passed on from the father to the first-born son, thus making him the father-priest of the family. The father-priest was meant to lead the family in covenant worship.
Though this is the case, in the book of Exodus, things drastically change because of sin.
The Israelites were in Egypt for 400 years. In Exodus 4, God tells Moses to go to Pharoah and say: “Israel is my son, my first-born. Hence I tell you: Let my son go, that he may serve me” (4:22-23a). This shows us a few important points.
First, God is a father.
Second, what Adam is at the beginning of creation, namely the first-born human son of God (cf. Luke 3:38), so too, as a nation, is Israel.
Third, what the first-born son in the family was meant to be, namely a father-priest, so too was the nation of Israel meant to be to all the other nations.
Fourth, Israel, as first-born, is called to serve (“abad”) God, which connotes covenant worship and sacrifice, as it did with Adam in Genesis 2:15. This is made obvious when we see what Moses says to Pharaoh in Exodus 5:3: “Let us go a three days’ journey in the desert, that we may offer sacrifice to the Lord, our God…”
To see God’s plan more clearly, we hear God say, “Therefore, if you hearken to my voice and keep [“shamar” means guard] my covenant…You shall be to me a kingdom of priests, a holy nation” (19:5-6).
Notice, however, the conditional nature of the statement; there is a big “if” included. In rapid fashion we find out that they do not hearken to God’s voice and keep the covenant. Their sinfulness radically changes everything until the coming of Christ.
Moses, on Mount Sinai, receives such commands as: “You shall have no other gods besides me” and “You shall not carve idols for yourselves…you shall not bow down before them or worship them” and “You shall not commit adultery” and “You shall not make gods of silver to be with me, nor shall you make for yourselves gods of gold” (Exodus 20:3, 4-5, 14, 23).
As Moses is receiving these commands, the people are breaking them. “When the people became aware of Moses delay in coming down from the mountain, they gathered around Aaron and said to him, ‘Come, make us a god who will be our leader…’ Aaron replied, ‘Have your wives and sons and daughters take off the golden earrings they are wearing, and bring them to me…’and fashioning this gold with a graving tool, [Aaron] made a molten calf. Then they cried out, ‘This is your God, O Israel, who brought you out of the land of Egypt.’ On seeing this, Aaron built an altar before the calf and proclaimed, ‘Tomorrow is a feast to the Lord.’ Early the next day the people offered holocausts and brought peace offerings. Then they sat down to eat and drink, and they rose up to revel” (Exodus 32:1-6).
God says to Moses, “Go down at once to your people…for they have become depraved” (Exodus 32:7).
Moses comes down the mountain, breaks the tablets of stone on the base of the mountain and cries out, “Whoever is for the Lord, let him come to me!” (Exodus 32:26). The Levites come to Moses and obey his command to “put your sword on your hip, every one of you! Now go up and down the camp, from gate to gate, and slay your kinsmen, your friends and neighbors!” (Exodus 32:27) Moses then proclaims, “Today you have ordained yourselves for service to the Lord…” (Exodus 32:29).
Israel was in Egypt for 400 years, and they had fallen into the idolatrous worship of the pantheon of Egyptian gods and goddesses. God’s intent was not only to bring the Israelites physically out of Egypt, but to also get Egyptian idolatry out of them. The sacrifices they were to offer during the proposed three days’ journey were the very gods they had been worshiping under the form of various animals. Moses tells Pharaoh, “…we shall sacrifice to the Lord our god offerings abominable to the Egyptians” (Exodus 8:26). In sacrificing these sacred animals, they would have been renouncing their false worship.
However, in Exodus 32 we find out the Egyptian ways are still in their hearts. The golden bull represented the Egyptian god Apis, the god of wealth, fertility and power. The worship of this god involved sacrifice, a meal signifying communion with that god, and orgiastic sexual worship. “Rose up to revel,” is a euphemism for this impure sexual worship.
With this comes a change in the priesthood. The Levites are now those who are ordained to serve the Lord. In particular Aaron, who is a Levite, and his sons will be the high priests, the ones who offer sacrifices. Now there are laws for animal sacrifice commanded, morning and evening, and many other times for many different situations, because of their idolatrous ways. In particular, once a year, on the Day of Atonement, Aaron and those in the priestly line of Aaron after him will have to sacrifice a bull, because of the sin of Aaron.
The letter to the Hebrews tells us, “For when there is a change in the priesthood, there is necessarily a change in the law as well” (7:12). In salvation history, this is where the book of Leviticus fits in.
God sees that it is going to take much more than a one-time sacrifice to get Egyptian sinfulness out of their hearts. He is providing them the opportunity to renounce and repent of their sins day in and day out.