Some things to think about:
Cain and Abel had different occupations, but both felt the need to bring an offering of some type to the Lord. Why did God accept Abel's offering but not Cain's? Some say it was because the offering was supposed to be a blood sacrifice. However, other grain and drink offerings seem to have been acceptable in certain contexts, so that is probably not the whole picture. Others point to each individual's character. Abel is said to be a righteous man(Matthew 23:35), but Cain shows by his immediate response of anger that something was already wrong in his attitude and in his heart. Still others point out the subtle yet pointed difference in the quality of their offerings: Cain brought 'some of the fruits of the soil', while Abel brought 'fat portions from some of the firstborn of his flock'. (Genesis 4:3-4 NIV)
Hebrews 11:4 (NIV) gives us some insight into this question of why Abel's sacrifice was accepted and Cain's was rejected: "By faith Abel offered God a better sacrifice than Cain did." This implies that Cain's sacrifice was done without faith. Maybe he was just going through the motions. Nevertheless,
- God cared about Cain's spiritual condition. In Genesis 4:4-7, we see that God notes the angry, downcast attitude of Cain and warns him of the dangers of sin, which crouch right at his door, ready to overcome him. What a vivid picture of the serious and destructive nature of sin! God also gives Cain fair warning of the consequences of continuing in this path, yet holds forth the way of escape from this attack. Remember, God already knew the murderous anger which was probably beginning to build in Cain's heart against Abel.
- Cain almost immediately does the exact opposite of what God suggests. He gives in to his angry emotions and, in a fashion which seems all the more horrible because of its thin veneer of brotherly affection and innocence, tricks his unassuming brother into accompanying him to an outlying field, where he kills him. Then Cain follows up this act with what can best be described as disrespectful indifference when God inquires as to Abel's location. This is made doubly horrific when we consider that Abel was a keeper of flocks of animals, and that Cain has not only subtly disdained Abel's honest profession, ('Am I my brother's keeper?' (Genesis 4:9 NIV) but also treated Abel as though he was of less value than an animal from his flock, which would have been sought after if it had gone missing. Of course God knew where Abel was all along, but He was giving Cain an opportunity to repent. God is continuing to reach out to this wayward son.
- Unrepentant Cain is banished. He can no longer be a worker of the soil, because he is under a curse and the ground will no longer yield its crops as it had before. Instead, he will be a restless wanderer on the earth. Cain believes himself to be hidden from God's presence and soon to become a victim of a revenge killing. God graciously puts a mark upon Cain so that no one would kill him. Then Cain goes out and lives in the land of Nod, east of Eden.
- Nod means 'wandering', yet Cain begins to build a city (v.17). He and his wife are expecting a child, and perhaps he believes he can overcome his sentence of being a restless wanderer on the earth by this project. You'd think he'd know better, but he is certainly not the only human to try to avoid the consequences of sin by engaging in a flurry of activity. It is interesting to note that those who persist in sin are often pictured as restless and unsettled. For example, Jude 1:12-13 portrays the wicked as clouds being blown along, as wild waves of the sea and as wandering stars. Isaiah 48:22 (NIV) says, "There is no peace," says the LORD, "for the wicked." In contrast, Psalm 119:165 (NIV) states: "Great peace have they who love your law, and nothing can make them stumble." Not that believers don't have struggles -- just that they have a 'peace which transcends all understanding' (Philippians 4:7 NIV) even while the storms rage around them.
- Where did Cain get his wife? He may have had this wife before he killed Abel and was banished. Otherwise it is hard to imagine any woman being willing to align herself with his cause. As you see from Genesis chapter 5, Adam and Eve had other children besides Cain, Abel and Seth, and these also had numerous children. We aren't told how old Cain was before he married, but apparently he married either a sister or a niece. The restrictions against such behavior do not come until later in the Bible. Nowadays marrying such a close relative is prohibited, mainly because of the greatly increased possibility of passing on harmful genetic mutations. Adam and Eve, though, were created without such problems and the effects of mutations in the following generations would take time to accumulate. Therefore, it would probably have been acceptable to marry a close relative. This still sounds a bit strange to us, but, if we take the Genesis account as factual, we have to realize that later, after the flood, the only humans alive were Noah, his wife, his three sons and their wives. Therefore, all humans are from Noah's family, whose members would necessarily have had to marry among themselves to continue. So I guess we all have a 'skeleton relative' or two in our own closets. Hopefully not literally.