God created the world very good. We don't know all the details--or, really, hardly any of the details--of that original world, but we do know that it was very good and there was no death in it. No person or animal died in that world and, if God's plan had continued, would they ever have died. But man sinned and that changed everything. It is for that reason that death entered the world.
The story of sin begins with a snake in the garden, a snake which enticed mankind to turn from God and commit the first sin. After that first sin, God came down and pronounced judgment against the snake, against woman, and against man. The judgment against the snake is found in Genesis 3:14-15. “And the LORD God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life: And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.”
There are two parts to this judgment or curse. First there is the degradation of the serpent--that it would be cursed above all the cattle and that it would a low-down snake-in-the-grass, essentially. Second, there is an enmity between the snake and the woman and between her offspring and its.
But the progress is very interesting. First there is an enmity between the snake and the woman. Second, there is enmity between the snake's offspring and the woman's offspring. This seems logical enough. And certainly, the human race and the snake-kind cannot be said to be on amicable terms to this day. “Ancient Israelites, who often encountered snakes in their daily activities..., would find the statement quite meaningful as an explanation for the hostility between snakes and humans... This ongoing struggle, when interpreted in light of v. 15, is a tangible reminder of the conflict introduced into the world by the first humans’ rebellion against God.” (NET Bible)
But there is a third step to this progression or, if you will, a third part to this curse, and that's the interesting part. And that is the conflict between the woman's seed and the snake itself. (Expositor's Commentary) “It (the woman's offspring) shall bruise thy head (the snake's), and thou (the snake) shalt bruise his (the woman's offspring's) heel.” This word used for bruise here refers to repeated attacks, attacks over and over again. The Word Commentary translates this couplet: “He will batter your head/and you will batter his heal.” There is an ongoing conflict here; not a single, one-time event which is very strange if it is between this particular snake and the woman's offspring. You would expect this conflict to be between the snake's offspring and the woman's offspring--but it isn't. It is as if this particular snake were going to outlive the woman and be the principle actor in a conflict which would go on for ages. “This suggests that the author views the snake in terms that extend beyond this particular snake of the garden... Consequently more is at stake in this brief passage than the reader is at first aware of. A program is set forth. A plot is established...” (Expositor's)
But if there is to exist enmity between the snake and the woman's seed, it has to be something more than an ordinary snake. And since it could talk and engage in subtle discourse, this seems clear anyway. But in order to find out who the snake was, you have to go the opposite end of the Bible. “And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.” (Revelation 12:9) Why does John bother to call this being both a dragon and a serpent? It makes sense as dragons are usually portrayed as reptilian, but in that case why bother adding the term serpent? And why does he call him that old serpent or, as some translate it, that ancient serpent? Clearly this John's way of connecting Satan with the serpent that tempted Eve. Whether Satan took on a reptilian form or possessed a serpent or however the scheme was actually worked, it was Satan who was the active agent in leading mankind into sin. He told the first lie which is why Jesus said that he is the father of lies.
So there is enmity between Satan and the woman's seed. If we see this simply as referring to humanity (since all humans are the descendants of Eve) there is definitely a truth here. Satan has been at continual war with humanity since Eden. And though there are those who explicitly follow Satan, for the most part even sinners hate Satan even as they follow him. (Wesley) And whatever you understand by the offspring of Satan, the truth also holds, since all sinful things, all things which spring from the Devil and his actions, are at war with humanity. Ultimately, even, in every man's soul, the offspring of the devil and the offspring of the woman are at war.
But there is another way to look at this passage. The offspring of the woman is referred to as singular. This is common when dealing with a collective group of people--but it is also possible to interpret this as a single entity--in other words, that it is one particular person who is described as the seed of woman, not the entire human race. Moreover, many have pointed out that it is odd that the offspring is described as coming from the woman. Both Adam and Eve were deceived by the serpent; they were, together, the parents of the human race and so the quarrel between them and the serpent (whether literal snakes or Satan) would be carried on by the seed of both. So why is it specifically the seed of woman? We may find the answer to that in Isaiah 7:14 “Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.”
The idea of their being a Messianic prophecy in this verse goes back to BC Jewish writings (Word) and has been reiterated by Christian writers since Iraneaus. (NET Bible) For this reason, this passage is sometimes referred to as the Protoevangelium, or First Gospel, since it is here, at the very point of man's sin, that the first hint is given of the deliverance from sin.
All death came into the world because of sin. The world is the twisted mess that it is because of sin. That is why when God promised to send a Messiah it was specifically to deal with this primal problem of Sin and its primal author, Satan. If the Messiah had been what the Jews expected, simple a political champion who would overthrow the government of Rome, He would not have solved the real problem. Rome was a recent problem and one which, one way or another, would pass away eventually. To be a true savior of the World, the Messiah would have to go back to the beginning and destroy the evil of the world at the root. And so it is prophesied that he would batter the serpent's head and the serpent would batter his heel. Though continual and mutual warfare is pictured here, it portrays a bad outcome for the serpent. “[T]he serpent is in a tactically weaker situation, being able only to strike at man's heel, while man can crush its head.” (Word) In every step the Messiah took, he was crushing the serpent's head while Satan could only, in futile anger, strike at his heel. But there is even more. The NET Bible says that it is possible to interpret the ending phrases of v. 15 as synchronous, so that it could be translated: “he will crush your head as you attack his heel.” In this case, this seems very possible a reference to the death of Christ--in that moment, Satan struck at the Messiah, lodging all his poison in His heel, only to find that by that act, he had affected his own defeat, for in the Crucifixion the master and final stroke of Sin is absorbed and transformed into the master stroke of God.
ReferencesExpositor's Bible Commentary, Vol. 2, p. 55
NET Bible, Genesis 3:15
John Wesley's Notes on the Old and New Testament, Genesis 3:15
Word Bible Commentary, Vol., 1, p. 80