After the fall, God could have left humanity mired in its guilt, but instead he promised his promised (Gen. 3:15) and conformed it by replacing their own cover-up clothes with the skins of animals that he had sacrificed for them (Gen. 3:21). It was no longer sufficient to bring the thank offering now. The guilt offering was also needed, the worshippers acknowledge of guilt and the divine provisions for its transfer to the sacrificial animal---as faith in "the lamb of God, who takes away the sins of the world" (John 1:29). Now we can see the significance of the religious war between Cain and Abel. "In the course of time Cain brought to the Lord an offering of the fruit of the ground, and Abel also brought of the firstborn of his flock and of their fat portions. And the Lord had regard for Abel and his offering, but for Cain and his offering he had no regard" (Gen. 4:3-5). And the rest, as they say, is history.
Jesus identified Abel as the first martyr for the faith (Matt. 23:35). We are also told, "By faith Abel offered to God a more acceptable sacrifice than Cain, through which he was commended as righteous, God commending him by accepting his gifts. And through his faith, though he died, he still speaks" (Heb. 11:4). By offering the guilt sacrifice, Abel was acknowledging his transgressions and his trust in a redeemer. His act of making the offering did not justify him, but it witnessed to his justification that he possessed through Faith in Christ. And that is why, "through his faith, though he died, he still speaks." Every time he brought the acceptable sacrifice, he was proclaiming the gospel, and does so still today every time his story is told. It is not going too far to suggest that Cain, by contrast, was the first "Pelagian." Cain might perhaps pay his membership dues (the thank or tribute offering) in the Covenant of Creation, but he failed to acknowledge that he stood guilty of violating that Covenant and needed to be transferred into the Covenant of Grace. Cain did not believe that a bloody atonement offering for his guilt was needed. This is the oldest and most perennial heresy, the default setting of our fallen heart. It may seem reverent for us to tip our hat in perfunctory gratitude to God, perhaps even tithings to the church and performing various duties, but we innately resist acknowledging our radical guilt and our need for God's equally radical forgiveness.
