Genesis 4:7

"If thou doest well, shall it not be lifted up? and if thou doest not well, sin coucheth at the door; and unto thee shall be its desire; but do thou rule over it."

This is one of the most difficult and disputed verses in Genesis, the problem being the identity of what is referred to in "sin lieth at the door." The usual theory that "sin" is here characterized or personified as a "savage beast," or a "wild demon" about to spring upon Cain, and that God was warning him to rule over the "sin" and thus refrain from committing it, has nothing whatever to commend it. The word for "sin" in this passage means "sin offering, a common meaning of the word in Scripture, as in Hosea 4:8; 2 Corinthians 5:21; and Hebrews 9:28."[14] This understanding of the passage is ancient. Clement of Rome, quoting the Septuagint (LXX) (which of course is incorrect), nevertheless correctly concluded that something was wrong with the sacrifice.[15] Understanding "sin offering" as the thing mentioned here strongly reinforces the necessary conclusion that the institution of sacrifice was already established and that God had laid down certain rules with reference to it, which rules Cain violated. The fact that many "moderns" deny this is no problem at all; the glaring evidence is right here. Adam Clarke wrote, "I have observed more than a hundred places in the O.T. where the word here is used for sin offering";[16] and there is positively no reason whatever for understanding it differently here. To borrow Clarke's paraphrase of what God said, "An animal proper to be offered as atonement for sin is now couching at the door of thy fold."

Thus, the great sin of Cain was simply this - he offered to God what he supposed would be just as good as what God commanded. He was the first innovator.

THE FIRST INNOVATOR

It is not accidental that the first innovator was the first murderer and that he founded the wicked generation that eventually corrupted the whole world. The innovators, or changers, of God's instruction always attempt to justify what they do. No one can show anything wrong with Cain's offering, except that it was Cain's idea, instead of God's. With all the specious logic of modern innovators, Cain might have tried to justify his action thusly:

If God wants smoke, my haystack has that fuzzy lamb beat a hundred ways. If God wants value, my wheat will buy fifty lambs. And all that messy blood; I never liked that anyway! God can save us if we never go near a drop of blood. Surely, God doesn't care about a thing like that; It's the spirit of the thing that counts anyway!

One may say that Cain would never have spoken like this, but his descendants do. And there is every reason to suppose that he fortified his disobedience with the same sort of rationalizing that men today use to defend their sinful tampering with the laws of God.