Why Cain's offering was not acceptable
There are those who think that Cain's offering was not acceptable because it was not a blood offering. Others think that it was not acceptable because it was a vegetable plant offering. However, many types of offerings were considered acceptable, and not all of them were blood offerings, consequently, I think that there was not a special rejection from God of offerings in which blood was not present.
This reasoning can also be applied to the second supposition, which affirms that Cain's offering was not acceptable because it was the product of plants and not of animals. One of the offerings of the day of Pentecost and first fruits offerings was wheat grain. Therefore, there was nothing against these kinds of offerings.
According to the narration in Gn 4: 3-5 we get the idea that Cain's problem was a lack of love and respect for the Lord God. When Abel's offering is described the focus is in the lovely way in which he chose an offering for God. He chose the best he could find, he brought the firstlings of his flock and the fat thereof. This tells us that he had great respect for God and that he loved Him enough to give Him the best, instead of keeping it for himself.
The attitude of Cain was just the opposite. He didn't chose the best, he just grabbed anything; as a person who wants to fulfill an obligation. He didn't look for the best of his crop; he thought that anything was good enough for God. His attitude was not that of a man who has the offering recipient in great esteem, respect, and love. This is why his offering was not worthy.
"And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering, but unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell." ( Gn 4: 3-5 )
Something similar to this is told in Malachi 1: 8 in respect to the
offerings the priests were sacrificing. They didn't look for the best for God,
and because of that God did not accept the offerings they offered in the
"And if ye offer the blind for sacrifice, is it not evil? And if ye offer the lame and sick, is it not evil? Offer it now unto thy governor; will he be pleased with thee, or accept thy person? saith the LORD of hosts." ( Malachi 1: 8 )
What Cain and Abel did at that time was not a ceremony of sin repentance, or an offering for their sins. That is why they did not have to offer necessarily a blood sacrifice. What Cain and Abel did was an act of worshiping in which each one chose what he supposed God deserved. That is why Cain's offering was rejected.