Genesis 4:2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
4 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:
5 But unto Cain and to his offering he had not respect.
We are told Cain was a farmer & Abel a shepherd. They both brought legitimate offerings to the LORD. Some have taught God only respected Abels offering because it was an animal. Not true. There were grain offerings set up by God. Over time, God respected, or recieved Abels offering but rejected Cains. Why? Hebrews tells us it was all about faith, and the lack thereof with Cains. We are to bring our offerings to the LORD with joy by faith. Without faith, its impossible to please God. The offering really isnt the object, its our faith in the LORD. Now, thats truly amazing because I realize I dont have much to offer God, but I do have faith. Now, you may have faith the size of a mustard seed, but that is powerful enough to move mountains. God would have recieved Cains offering if he brought it with faith, as I imagine he had probably done in times past. LORD forgive me my unbelief, help me believe always.