It is interesting that Abel’s name literally means a little breath or gentle wind. But it also has the connotation of the word vain or vanity. In fact in Hebrew, Abel’s name is the word translated vanity in Ecclesiastes as Solomon says, “Vanity of vanities, all is vanity” (Ecc 1:2). The NIV uses meaninglessness there. The Holman and NET have futile or futility. And the idea is that there is not much of importance there. Was this a name given at birth or was this a name that became associated with Abel because he was thought of as one of little consequence.
If Abel truly was little thought of, the fact that God looked favorably on him and rejected the more accomplished Cain could have added to Cain’s feeling of being affronted. Whatever the case, Cain was very angry with the rejection of his offering. The Bible uses a word that means burning, smoldering, hot in his anger. And his face had fallen.

Now, we know why God did not accept the offering. It can be summed up in what we usually think of as a faith versus works idea. Abel’s offering demonstrated his faith in God for God’s TGB in which he could be satisfied in life—in relationship. That dependence in God was his hope in life, and faith is the  substance or basis of our hope. Cain, on the other hand, attempted to do it on his own—the works mentality. Recognize that works is not just anything you do. As we read in James, acting in ways that demonstrate your faith is natural and expected. But works has become a shorthand way of describing the pursuit of TGB without God, and Paul uses it that way, for example, in Romans 4:2 and 9:11. So Cain’s offering was one of works, not showing dependence on God but rather being impressed by and trying to impress God with his gift.
And so Cain was furious that God was pleased with this nobody Abel and his gift but snubbed Cain and his gift.
But God explained to Cain that he did not do well. He was not true to the covenant—to relationship; he was disingenuous to the point of the offering. God used a little play on words in the Hebrew to express that. He asked, “Why has your face fallen?” and then said, “If you do well, you will be lifted up.”

But then God says something that many people understand to be a warning. We read in verse 7 in the Holman: “If you do not do what is right, sin is crouching at the door. Its desire is for you, but you must rule over it.” The common interpretation for this verse is that sin is being personified here. So God is saying that sin is crouching, always ready to pounce on us. Sin wants to make us do its evil deeds, but we must not give in—we must master its impulses.
I think there are a few problems with that interpretation. The first is that the noun sin here is feminine in the Hebrew. But the participle crouching and the pronoun it, in the next sentence referring back to sin, are masculine. Didn’t Moses understand the grammatical relationship? How did he make such a grammatical error? It doesn’t fit.

A second problem is found in the first part of the verse reads, “If you do not do what is right.” Not doing what is right—missing the mark of doing right—is doing wrong; it is sin. So, if we substitute the word sin for the phrase, what is God trying to say: “If you sin, then sin is crouching at the door”? Why would sin be crouching at the door ready to strike if you’ve already sinned? What does this sentence even mean?

The third problem is that it seems as if God is saying we can avoid sin by just trying harder to be good—to do right. Is that what God’s saying? Surely God doesn’t believe that, does he? God himself in Christ had to come as the man Jesus precisely because we could not possibly do what this verse seems to encourage us to do. Sin’s hold in our flesh so influences us to sin that Paul declares there is not one who has overcome that temptation except Christ. We all sin.

But even though we know that, we look at this verse and shake our heads at Cain, wondering how he can be such a terrible person to sin, while he half assume that Abel and Seth and others (and ourselves) are all better. No, the fact is we’re not better. In fact, that was exactly what Abel was saying with his offering. He said, “God I can’t do it. I depend on you!” Therefore, I doubt very much that God actually wanted, in verse 7, to tell Cain just to try harder.

I think our first corrective in trying to understand what is being said is to focus on that word sin. It, after all, has created the grammatical error. So let’s try to fix that first. The word here translated sin occurs in the OT about 271 times. And about half those times it is translated just as that—sin. However, a whopping 109 times (mostly by Moses in the Pentateuch) it is translated as “sin offering” because it is being used as an actual thing rather than the concept of sin. When the Bible says someone brought a sin offering to the altar, the Hebrew looks like it is saying someone brought a sin to the altar. But it means a sin offering, and so translators have correctly called it “sin offering.” However, even if we change sin to sin offering, although we fixed the grammatical problem, we still have difficulty understanding the sentence. It needs more fixing. Our only advantage so far is that we can have a masculine subject in the sin offering that corresponds to the masculine participle and pronouns that follow.

But why would a sin offering be crouching, ready to pounce? Actually, the Hebrew word translated crouching simply means lying down, almost always used for an animal lying down. For example, we are familiar with the imagery in Psalm 23 where the Lord as the shepherd of David (the sheep) makes him “lie down in green pastures.” And in fact, although every major modern translation uses “crouching” in Genesis 4:7, none of them ever translates it as anything but a synonym for lying down (reclining) anywhere else in the entire OT. So why translate it as crouching here? The translators are prejudiced by the commonly understood interpretation of this verse with sin ready to pounce. They figure that sin couldn’t simply be lying down. Sin is evil. So sin must be crouching, waiting to attack. Yet that’s not the Hebrew word, and so we shouldn’t change the word to force the interpretation. By translating the word as we would normally (and have done everywhere else it is used), we now have so far “a sin offering is lying down at the door.”

Even with two changes to the text, we are still left confused. What is a sin offering doing lying down at the door? What door? Let’s do a little background check. Remember in chapter 3 after Adam and Eve sinned, they were expelled from the Garden of Eden. And then we are told that the way to the Tree of Life (in the Garden) was being guarded by a cherub on the east side of the Garden. So the Garden was constructed with its entry on the east. Now, after being expelled, where did Adam and Eve go? Did they pack up and move to, say, Canada? Probably not. Most probably, they settled down right there outside the Garden to the east.

Cain and Abel probably had the idea of giving offerings to God because they had seen their parents do the same thing. Now, if you were Adam or Eve and you wanted to take an offering to God and you knew that you had been dismissed from the presence of God by being removed from the Garden (where you had formerly enjoyed relationship with God), where would you bring your offering? I’m guessing they brought their offering back to the Garden—or very close to it. Of course, they could not go into the Garden after being cast out, so they went as far as they could and offered it right there at the entry, the gate, the door of the Garden.

Now, is this wild conjecture on my part to assume that was the case? Really, no. This eastern-facing Garden which housed (so to speak) the presence of God is exactly the template for the tabernacle and temple which were eastern-facing structures to house (so to speak) the presence of God. And because the Israelites could not enter the temple, they brought their offerings to the eastern gate of the temple or tabernacle. So we see a definite pattern set up by the Garden for the later sacrificial system conducted in Israel with the temple.

Now, for a moment, let’s talk about sacrifices. Biblical sacrifices do not have the same purposes as those brought by the non-biblical fallen world. The sacrificial objects in both cases are things that are treasured. But the purpose for the sacrifice of the false worshipper is either to gain the favor of the god (imploring the god for rain for the crops or something like that) or to appease the god if he is angry for some presumed offense that the people have caused. But neither of those purpose is the point of biblical sacrifice.

The Bible teaches that its instructed sacrifices have two purposes: (1) to signify the sacrificer’s desire for righteousness—dependence on God for life, not on physical essence—and (2) to signify the sacrificer’s desire to return that which had been cursed (separated) to a state in which it could then be accepted. That second purpose needs more development.

Adam and Eve were possessors and rulers of this physical creation. God gave it to them and put them in charge. In their sin, they separated that essence from God—they caused its curse. They caused its death. OT sacrifices normally show the desire of the worshipper to take this physical essence he or she owns and controls, put it to death in relation to self, and have it transformed (or brought back, redeemed, given) to God. When we see the sacrifice killed and then burned and find the smoke rise to heaven and hear proclaimed that it is a sweet savor to God, we see this whole process take place—the return of cursed essence from death to acceptance (life) with God. That is the basis for resurrection and restoration.

Of course, the great ultimate sacrifice in the NT was that of Christ. He lived his life in dependence on God; he was righteous in regard to the Covenant of Life. When he laid down his physical life, he was putting his essence to death, demonstrating the end of that cursed existence. But because his spirit was completely righteousness (faithful to the Covenant of Life), he who owned and controlled that physical essence could redeem it—reverse Adam and Eve’s movement of physical essence away from God—and move that physical essence back into relationship with God, with the sin curse gone.

Jesus did not do what many Christians falsely imagine. He was not conducting a sacrifice according to the non-biblical, false worshipper purpose to appease a wrathful god by suffering for the guilt of sins placed on him. Rather he suffered and died for our sin (our physical curse) so that our sins could be forgiven us through our faithful, righteous dependence on God.

Now, with all this background in mind, let’s return to our Genesis 4:7 passage. The first part of the verse reads: “If you do what is right, won’t you be lifted up?” And that is a correct translation. The if in the Hebrew can be conditional as here. However, it can also denote conceding as in though or although. In the second statement of this verse, I think the second connotation is intended. And along with this conceding, the verb should be understood in past tense. With our previous changes, it now reads: “But although you did not do what is right, a sin offering is lying down at the door.”
Now, finally, we have sense. What God was telling Cain (and teaching us) was about the second purpose for sacrifices—to put to death this physical essence upon which we had depended. God told Cain he had done wrong in his first offering which should have been intended to show dependence on God. He had not shown that dependence. And for missing the mark (sinning) in that first offering, God was now suggesting he bring a sin offering (sacrifice) to display his desire to put his fleshly passion to death and return to God. And prefiguring what God would do in Christ (provide the sacrifice for us), God told Cain that he had provided a sacrificial animal (the sin sacrifice) and had it lying down at the place of sacrifice (the door or entry to the Garden).

The final statement of the verse corresponds to the picture of the sin sacrifice. Although through Adam and Eve’s sin they had given themselves up to the control of the flesh and its fleshly desires, the sacrificer should show the will to judge that flesh, sentence it to death, and return the life back to relationship with God.

So in this passage—this whole story of Abel and Cain—God has been intent on teaching us the two necessary elements of the restoration process. Abel’s faith offering showed the desire by the image bearer to follow the Covenant of Life obligation to depend on God for his provision of TGB. And Cain’s potential sin offering was to show the removal (putting to death) of the cursed physical essence that leads us to sin.