After Adam sinned and realized he was naked, he sought out fig leaves to cover himself. God did not accept the covering that Adam found for himself; rather He provided His own covering for him, one of animal skins. We learn through Revelations that the animal skins represented, even at this earliest time, the sacrifice that Jesus was to make for mankind. Jesus was "the lamb slain from the foundation of the world." (Rev. 13:8).
Animal sacrifice became a part of the "school master" which was to lead us unto Christ (Gal 3:24). It exhibited the divine principle of redemption that God would require, before he would forgive us our sins.
Blood represented life. On the altar, the blood was poured out after a confession of sins by the offerer. Sacrifice, then, exhibited the pouring out of life (or death) because of sin. The person making such a public declaration was showing that he recognized God to be right and just in requiring death as the consequence of sin.
In performing this, the offerer showed that he recognized the wickedness of sin (that sin was worthy only of death) and he expressed his desire for atonement. Atonement is easily understood by breaking up the word: It is "at-one-ment." In the Garden of Eden, man ceased to be at-one with God as the consequence of sin. Atonement is the expression of a man's desire to heal the breach created in the Garden, and to regain the privileged condition he had with God at that earlier time.
The "seed of the serpent" (or sin) was to bruise the "seed of the woman" (who was Christ) on the heel, but Christ would bruise it on the head. A bruise to the heel is not fatal, but the bruise to the head is. Christ died because of sin, when sin bruised him on the heel, and God raised him. And when God raised him after he had conquered sin, that is, after he had conquered the will common to all mankind, he delivered the blow to the head of the serpent and was made immortal. Sin could no longer claim any hold on him. The serpent was dead!