SOMEONE RECENTLY ASKED A GREAT QUESTION ABOUT TORAH PARASHAH 1. The question was this:
Why was Cain's offering not accepted? He specialized in crop cultivation and his offering should come from the fruits of his field, right?
The very plain and simple answer to your great question is that YHWH handed down all kinds of commands for humans, right from the beginning – commands that weren’t mentioned in Torah (because if everything that happened during those thousands of years, the pages of the Torah would circumscribe the earth several times over).
But we can surmise they WERE given, as we see in the case of Cain and Abel.
YHWH must have, at some point, told the humans alive at the time that they were to offer the blood of an innocent animal as a SIN offering. Grain does not cover sin. Cain’s grain offering might have been accepted IF he has also brought the blood offering required to forgive his sins….
Scripture tells us that blood, the stuff of life without which we would die, is extremely important to YHWH (i.e. in Genesis 9:4, Leviticus 7:26; 17:12-14; 19: 26, etc.) … It is so important that, innocent blood (the life of an innocent animal) has to be used to atone for one’s sins. Basically, it’s one "soul" for another!
Leviticus 17:11 11 For the life of a creature is in the blood, and I have given it to you on the altar to make atonement for yourselves; for it is the blood that makes atonement because of the life.’ 12 This is why I told the people of Isra’el, ‘None of you is to eat blood, nor is any foreigner living with you to eat blood.’ (CJB)
YHWH even explained the importance of blood back in Genesis 9, after He told humans that it was okay to eat animals (which is the reason why animals started fearing men!).
Genesis 9: 3 Every moving thing that lives will be food for you; just as I gave you green plants before, so now I give you everything — 4 only flesh with its life, which is its blood, you are not to eat.
5 I will certainly demand an accounting for the blood of your lives: I will demand it from every animal and from every human being. I will demand from every human being an accounting for the life of his fellow human being. 6 Whoever sheds human blood, by a human being will his own blood be shed; for God made human beings in his image. (CJB)
Cain’s grain offering (a type of "thank" offering) was invalid because it was not preceded by a blood offering, acknowledging sin. Torah tells us that ONLY BLOOD atones for sin, because "life is in the blood." Cain was basically "approaching the Throne" without a covering for his sin.