“Yahweh God made coats of skins for Adam and for his wife.” God had told Adam that when he ate of the fruit of the tree of the knowledge of good and evil he would die (Gen 2:16-17). Adam and Eve did not die the day they ate from the tree, but something did die; animals. The animals were most likely sheep or lambs—we will assume that for the purpose of this commentary—and God used their coats to clothe Adam and Eve. Everything in the Garden of Eden ate plants at that time (Gen. 1:29-30), so no animal was being killed for its meat (humans did not eat meat until after Noah’s Flood; Gen. 9:2-3). The sacrifice of the animals here in Genesis 3:21 was likely twofold: to provide Adam and Eve with proper coverings and also to make a substitutionary sacrifice that would atone for their sin before God, just as the later Levitical sacrifices atoned for sin (Lev. 1:4; 4:31, 35).
From what the Bible tells us about animal sacrifice as a covering for sin, and from knowing that Jesus, the “lamb of God,” died for our sin, it seems logical to conclude that God postponed the death of Adam and Eve and sacrificed an animal in their place to atone for their sin. Had Adam and Eve died the day they sinned, then God’s plan for a human race would have come to an end, so it makes sense that God would have planned for a way to save the human race and point to the Redeemer of the human race both at the same time, which an animal sacrifice did. The animal sacrifices that temporarily covered sin ultimately pointed to God’s great act of mercy in commuting the death sentence and granting everlasting life to everyone who accepted the death of God’s Son, Jesus Christ, in place of their own death.
More evidence that at least part of the reason God killed the sheep was as a sacrifice to atone for sin is that the godly practice of sacrifice had to start somewhere, and the most likely place would be God’s example in the Garden of Eden. We see by Genesis 4:4 that Abel brought a sacrifice to God from his flock, but what kind of sacrifice could an animal be at a time when people did not eat meat? It almost certainly would have been some kind of burnt offering, which would have been burnt on the altar (cp. the burnt offering; Lev. 1:5-9). It is hard to imagine that the practice of godly sacrifice could start any other way than God establishing the practice Himself. After all, it would not seem logical that a sinful person could be made right in the eyes of God by killing an innocent animal. It is not logical that someone would think, “I have sinned, but I can become right in God’s eyes by killing an animal.” How could the death of an innocent animal atone for the sins of a human being? The idea of animal sacrifice to atone for human sin had to start with God. God would have known His long-term plan and that He would redeem humankind from sin by the death of a sinless human being. Thus, God would have seen the value of setting forth an example of how the death of one (an animal or sinless person) could atone for the sin of another person; and God made that example concrete by setting forth the practice of animal sacrifice. But no human would have known God’s plan of redemption, and no human would have thought that the death of an animal would atone for human sin.
Given that the idea of sacrificing an animal to atone for human sin had to start with God, it is likely, but unstated, that after God sacrificed animals for Adam and Eve, that they themselves then presented offerings and sacrifices to God, and that is where Cain and Abel would have learned about it. It is unlikely that God started the idea of a proper sacrifice with Cain and Abel, or that God had somehow personally or through an angel taught them about sacrifice and the proper way to do it. The idea for a sacrifice that would atone for human sin via the death of a sinless Savior was already in the mind of God before Cain and Abel were born (Gen. 3:15; 1 Pet. 1:18-20; Rev. 13:8), and He started the idea when Adam and Eve sinned and they would have passed the idea down to their children, which would include Noah. The fact that Noah practiced animal sacrifice (Gen. 8:20) explains why almost every ancient culture practiced animal sacrifice, although as time passed the practice became quite perverted in many cultures.
[For more on the death of the animals atoning for the sin of Adam and Eve, see commentary on Gen. 2:17, “in the day”].