There are a couple of firsts in this chapter. The first children are born, but there is also the first murder. We haven’t been told of any rules God has put in place since the Garden of Eden, so it’s not clear why Cain’s offering was rejected. Many find it significant that Cain brought some fruit, while Abel brought the firstborn or best of his flock. Maybe Cain was bringing produce that wasn’t good enough to eat. Whatever was wrong with Cain’s offering, God gave him the chance to make it right. Like Adam and Eve, he had a choice. Doing the right thing would have resulted in happiness; doing the wrong thing resulted in him coming under the control of evil.
A question that often comes out of reading this chapter is where Cain got his wife. The answer is found in the next chapter, which we will not be reading. In verse 4, Adam is said to have had other sons and daughters. Cain must have married a sister or niece. That would be unacceptable today, but apparently acceptable in that time.
We are shown we always have a choice: to do the right thing or the wrong thing. Doing the right thing allows us to experience happiness and a closer relationship with God. Doing the wrong thing leads to doing things that are even worse, yielding damaged relationships with God and other people.