The story in Genesis concerning the offerings of Cain and Abel do not specifically tell us why God had "favor" for Abel's offering, yet did not look with favor on Cain's. Each man had given God from his occupation. Abel kept flocks and Cain farmed. So, I have to look into the story as an outsider who is trying to understand. Often, the best way to do this is through questions rather than answers.
Hebrews 11: 4 says that Abel's offering came through faith. It does not speak of Cain's. Should I assume that Cain's was not from faith if his is not mentioned? Hebrews 11 is a chapter of the Bible indicating that faith will result in actions. It commends those who have acted by faith. It states that faith is "being sure of what we hoped for and certain of what we do not see."
Leviticus tells us that God has required three types offerings. They are the guilt offering, burnt offering and peace offering. Each was distinctive.
The guilt offering was given to absolve the guilt of the person. This offering pointed toward the sacrifice of Jesus. It is remembered today through the confession of sins. We are forgiven by the blood of Jesus, no other sacrifice is necessary. Yet, the estrangement between us and God is met on a daily basis through the confession of our sins. We can still know Him and go to heaven but we will never have a close relationship with God while we are sinning. Confession corrects this so that we can walk with God daily.
The burnt offering was one of commitment. It symbolized the consumption of the offering on the altar.It was often given before an army would go to war so that they would make sure that their commitment was to the Lord. It is symbolic also of a passion to do whatever the Lord has asked.
The peace offering was one of grain. It symbolized the fellowship that one had with the Lord. It was shared within the family. It invited the Lord to come and commune with those who made this offering.
Abel brought an offering of the firstlings of his flock. It says that he also brought that of their fat portions. The firstling could have been for a guilt offering. The burning of the fat was used for a burnt offering. Is Abel's act of faith a confession of guilt before the Lord? Did he continue through a commitment to the Lord with a burnt offering?
Cain offered grain. Grain was used in a peace or fellowship offering. Was he saying that he wanted to commune with the Lord? Did he fail to notice his own sin? Does true communion mean we should first confess and commit ourselves before we can commune? Is this why God had no favor for his offering since he did not act in faith?
Many times that I have been in church without a moment of worship. I guess I thought it was the church's responsibility to make me worship. Worship is a communion with God. I thought I could worship without confession and commitment. I often fail to look at my own sins. I often commit my life to the Lord as long as it is convenient. I serve conveniently. I give conveniently. And yet I hope to worship in full communion with God. God won't stand for the worship of the lips when the heart is far from Him.
I really can't speak definitively why God rejected Cain's offering. I think I know. I think Cain was just like me. He expected to commune with God without confession and commitment. Abel knew better. God is always excited when people "get it."
Genesis 4:4-5 (NIV)
4 But Abel brought fat portions from some of the firstborn of his flock. The LORD looked with favor on Abel and his offering,5 but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
Hebrews 11:4 (NIV)
4 By faith Abel offered God a better sacrifice than Cain did. By faith he was commended as a righteous man, when God spoke well of his offerings. And by faith he still speaks, even though he is dead.