Abel was a shepherd, a profession often referred to for men of great passion for God. Moses and David were both shepherds, and our Lord called himself the Good Sheperd. Abel sacrificed in faith, in love. He gave his firstborn sheep, which he loved, and gave it to God. He understood what God had given him, and was penitent, knowing he was a sinner. It was not the sheep that pleased God, but Abel offering his heart.
Cain was a farmer, a sower of seeds. The Lord talked of sowing the seeds of God’s word into the hearts of men. Cain was not reaping a righteous harvest nor was he sowing seeds of God. His sacrifice to God was vain. God, in not regarding the sacrifice of Cain, recognizes the poor spiritual harvest. For God is not mocked, for whatever one sows, that also he will reap (Galatians 6:8) and Those who plow iniquity and sow trouble reap the same (Job 4:8). No doubt that Cain toiled over his efforts- his angry reaction tells us as much. It has nothing to do with what was offered, because God accepted grain offerings. It was not the amount, because the The Widow gave a small amount, but as it was all she had, Jesus said she gave more than the rich. While this is the first case of God showing no regard to an offering, it is not the last. See Isaiah 1:11-17, Amos 5:22, Hosea 6:5, These verses speak of God rejecting the sacrifices of Israel, because they were vain. From these examples, we know It is not enough to just go through the ceremonial motions. We know it is not enough to give a lot.
Many try to justify their lives and please God in their own way, rather than His way. God knew Cain had not given over his heart. But God does not tell Cain his life his hopeless now. God knew Cain was upset, and he called to him, not to ridicule him, but to instruct him on what he needed from him.
Sin waits to destroy us at our weakest moments. Yet if we put our faith in God and live by His word, our lives are protected from sin. This sentiment is echoed in the Epistle of James: Submit yourselves therefore to God. Resist the devil, and he will flee from you (4:7). Cain had not submitted himself to God, and he would not resist the influence of Satan, as God had warned.
Cain spoke to his brother– likely as a friend, to decieve him, and lure him into the field to kill him. Satan is a great deciever. God knows the number of hairs on our heads (Luke 12:27). He knew Abel was killed. The Lord offered Cain an opportunity to repent, but Cain denied knowing where Abel was, as if he could fool God. Am I my brothers keeper? Yes, you are. We are all responsible for each other. What have you done? These words awaken the conscience, spoken by our parents, spoken by our Father in heaven. The voice of your brother’s blood is crying to me from the ground- God loved Abel. He hears the cries of all his children. Cain is banished, and has to wander the Earth. To live without God is to wander, with no spiritual direction, and can easily lead to despair and sin. God had mercy on him, sparing his life, yet still Cain showed no repententance, which demonstrates the character of his heart. His first thought was only himself, not even a thought to his younger brother, a brother he should have protected and loved. God saves his people again and again, and yet still they turn from Him. The Lord marks Cain to protect him. We do not know what the mark was, and we should not speculate. Cain leaves the presence of God, never dedicating his life to Him.