2 Later she gave birth to his brother Abel. Now Abel kept flocks, and Cain worked the soil. 3 In the course of time Cain brought some of the fruits of the soil as an offering to the LORD. 4 But Abel brought fat portions from some of the firstborn of his flock. The LORD looked with favour on Abel and his offering, 5 but on Cain and his offering he did not look with favour. So Cain was very angry, and his face was downcast. 6 Gen 4:2b-6
Here begins the mysterious theme of the favour of God. Immediately every reader asks "why does God accept one offering and not the other?" It's a "works" based question, a "we get what we deserve" kind of question. Surely Abel's offering must be better in some way and sure enough many suggestions have been made* but the passage does not tell us why and I think that's significant in itself. Surely the writer knew that the question would be asked but we have to wait for the NT to get a clear answer to this question:
Heb 11:4 By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts. And through his faith, though he died, he still speaks. (ESV)
This little story about two brothers, each trying to relate to God, acts out the gospel of grace. If we look for some merit in their offering or their actions we will not find it, that's why it's called favour, but there is something that sets them apart. God's grace, his underserved kindness, is accessible by faith. Faith says "I have, no currency to pay you, nothing of value to barter with, I rely only on your gracious provision for me". Jesus told a similar story thousands of years later where two men attempt to relate to God in different ways. One, a traitorous tax collector, came with nothing but his septic sin and cried for mercy. The other, a religious Pharisee came with a list of shiny good deeds and boasted in his accomplishments. It's interesting that Jesus chooses Matthew, a tax collector, as his disciple, while the Pharisees plot to have Jesus killed.
Cain's name is related to the word 'get'(and the ESV study note tells me it sounds like the Hebrew word for 'gotten'). He is the first born, with everything coming to him. Abel's name could mean 'vapour' leading one commentator to ask 'who would name their son vanity, vapour, or nothingness'. It's a good question and while drawing out too much from the names can be a bit speculative, they do tend to mean a lot in the bible.
One of the most tragic stories of the bible ends in 1 Sam 4:21. News is brought from the battle field that Eli's two sons are dead (Eli is the second to last Judge) and that the ark of the covenant has been captured by the Philistines. On hearing this Eli falls of his chair backwards, breaks his neck and dies. When his daughter in law hears all this she goes into premature labour, names her baby Ichabod and dies. Ichabod means "The glory has departed from Israel!" 1 Sam 4:21. Some people get tattoos to say something powerful and lasting. Naming someone is even more meaningful and permanent.
Anyway, if names are important here, it could be that Cain, who comes to God with everything that man could offer, gets rejected but Abel who comes as nothing gets accepted. Coming to God like Abel is a real place of rest. I'm accepted because of something in the heart of God. It draws you in to focus on him and his love. It's not about us, it's about his love and his promises (Deut 7:7).
*Some suggestions are:
1) God likes animal sacrifices but both animals and grain are legitimately offered later (Deut 15:19-23, Lev 2, Deut 26:2)
2) God just prefers shepherds like Joseph (Gen 37:2), Moses( Ex 3:1) and David (16:11, 17:34)
3) The idea of first born is significant. First borns and first fruits must be offered in sacrifice or redeemed (Ex 22:28-29, 34:19-20)
4) Abel was good and Cain evil...