Question: Why was Cain’s gift not as good as Abel’s? How was Cain supposed to know the rules? I have never seen it in the first 3 chapters of Genesis. Is it possible that his sin was not his initial gift, but his reaction to God after he gave his gift?
Answer: There is no evident instruction recorded in Genesis about how offerings were to be made. This does not mean God did not give instruction, but that it was not recorded. The fact that it is not recorded lends strength to the idea that Cain’s offering was not rejected because he broke some rule (brought grain instead of an animal sacrifice, for example). Rather, it suggests that it was his attitude in bringing the offering that was so offensive to God.
We can imagine what the attitude was by examining our own attitudes. Sometimes we do things “for God” out of an attitude of hypocrisy (“I have to do this or everyone will think I don’t like God”), or anger (“I’m mad at God but I’m going to go through the motions of living as if I’m not”), or ingratitude (“I accomplished this but I have to make it look like I’m giving God credit”), or even jealousy (“I don’t like the fact that so and so is living for God and gets credit from others, so I’m going to compete”).
It seems that the reaction of Cain to God’s reprimand was a response to being exposed. His original bad attitude in making the offering was exposed when God did not accept it and then he got angry and perhaps depressed over it. That sounds like us, too, doesn’t it?