The first mass-redemptive covenant is the Mosaic Covenant that was delivered to Moses on the Mount Sinai after Israel had departed Egypt. Various other covenants were created before this, but I believe the Mosaic covenant eventually became the full realization of those; it was merely a formal establishment of a covenant that was already in place.
The use of animal sacrifices was seen very early on. Hebrews 10 tells us, “For the law, having a shadow of the good things to come, and not the very image of the things, can never with these same sacrifices, which they offer continually year by year, make those who approach perfect….For it is not possible that the blood of bulls and goats could take away sins.” (Heb. 10:1,4) The sacrifices of animals that we see as far back as Adam’s time were a foreshadowing of God’s perfect plan.
These sacrifices were signified by the leaves that Adam and Eve used to cover themselves originally. They provided a covering, but it was not perfect so God made for them “tunics of skin, and clothed them.” (Gen. 3:21) Now it’s interesting to consider here that God could have easily told them to make their own tunics. After all, labor was a part of the curse that was upon the human race a result of sin. But the writer of Genesis clearly points out that it was God who made the covering.
Notice the nature of this act: the leaves failed to provide a sufficient covering, so God Himself shed the blood of a living creature to produce proper clothing for Adam and Eve’s shame. Surely this pointed forward to a time when “it pleased the LORD to bruise [Jesus]…to make His soul an offering for sin.” (Isa. 53:10)
By the fourth chapter of Genesis we see the sons of Adam and Eve making sacrifices to God. At one point we see that Cain and Abel each offered different sacrifices—one offered fruit and the other offered an animal sacrifice. (Gen. 4:1-4) At this point, the symbolism takes on a new dimension. As we have seen the animal skins as a covering were the literal sufficiency of physical nakedness, and it was the vegetation that proved to be an inadequate substitute. Now, in Cain and Abel’s situation, they were working on a symbolic level. Therefore, it is the animal sacrifice that is insufficient and it’s Jesus’ sacrifice that is the spiritual efficacy; and vegetation has no place in the story. Even though we see grain offerings and the like under the Mosaic Covenant, animal sacrifices had a specific purpose for underscoring our need for a Substitute. Grain offerings were meant to display thanksgiving, while sacrifices were used when asking for forgiveness of sin. Of course then, this understanding would explain why God was displeased with Cain’s vegetable offering. (Gen. 4:5)
Cain’s failure highlights a prominent human tendency. In each covenant we will see those who try to enter the covenant by their own means. In our day, it is even common for believers to reduce the Gospel by identifying a work, or set of works, that solicit entrance into God’s covenant of favor. This modern evangelical typicality is constructed on an improper understanding of the covenant relationship.