And the LORD had regard for Abel and his offering, but for Cain and his offering he had no regard. – Genesis 4:4-5

Why did God accept Abel’s offering and reject Cain’s? Some have said that it because Abel offered blood and Cain didn’t. That won’t work because in the law there were commanded grain offerings, even as sin offerings. (Lev 5:11) There’s nothing in Genesis 4 that indicates the offerings they made were supposed to be a sin offering, it was most likely a fellowship offering. No, the problem wasn’t the offering, it was the offerer. Hebrews 11 says that “By faith Abel offered to God a more acceptable sacrifice than Cain”. By faith Abel did that. Cain lacked faith. And why did Cain kill Able? “Because his own deeds were evil and his brother’s righteous.” (1 John 3:12) God rejected Abel’s offering because it was made by a faithless, evil man not because it was made of grain.