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Genesis 4:2-7:
4:2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
4:3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
4:4 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:
4:5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
4:6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?
4:7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
Why wasn’t Cain’s sacrifice accepted?
God told him why:
Gen 4:7 – If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.
Cains sacrifice was not accepted because he was not doing well.
The Bible says:
Pro 15:8 – The sacrifice of the wicked is an abomination to the LORD: but the prayer of the upright is his delight.
Pro 21:27 – The sacrifice of the wicked is abomination: how much more, when he bringeth it with a wicked mind?
The problem was that Cain was a wicked man who was not doing well. His sacrifice was therefore not acceptable to God.
Abel was a righteous man and therefore His sacrifice was acceptable:
1Jo 3:12 – Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother’s righteous
Heb 11:4 – By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he was righteous, God testifying of his gifts: and by it he being dead yet speaketh
The Bible says:
Psa 51:17 – The sacrifices of God are a broken spirit: a broken and a contrite heart, O God, thou wilt not despise.
All this goes to show that a sacrifice or atonement does not save an impenitent man. There are many who think they are covered by the atonement of Christ while they continue to sin every day. The atonement doesn’t work like that. That’s the error of Cain.
Jde 1:11 – Woe unto them! for they have gone in the way of Cain
The atonement is not unconditionally saving in its nature. Only those who are broken over their sin and forsake their sin are covered by the Atonement of Christ.
Pro 28:13 – He that covereth his sins shall not prosper: but whoso confesseth and forsaketh them shall have mercy.
A BLOODLESS SACRIFICE?
The first theory I heard as to why his sacrifice was rejected was because Cain’s offering had no blood. But there were grain offerings in the Bible, corn offerings, flour offerings, along with gold, silver, brass, and wine:
Exo 30:13 – This they shall give, every one that passeth among them that are numbered, half a shekel after the shekel of the sanctuary: (a shekel is twenty gerahs:) an half shekel shall be the offering of the LORD
Exo 35:5 – Take ye from among you an offering unto the LORD: whosoever is of a willing heart, let him bring it, an offering of the LORD; gold, and silver, and brass
Exo 35:22 – And they came, both men and women, as many as were willing hearted, and brought bracelets, and earrings, and rings, and tablets, all jewels of gold: and every man that offered offered an offering of gold unto the LORD.
Lev 9:4 – “a grain offering mixed with oil”
Lev 9:17 – Then he brought the grain offering, took a handful of it, and burned it on the altar, besides the burnt sacrifice of the morning.
Num 15:5 – “And the fourth part of an hin of wine for a drink offering shalt thou prepare”
Num 28:28 – And their meat offering of flour mingled with oil, three tenth deals unto one bullock, two tenth deals unto one ram,
Neh 10:39 – For the children of Israel and the children of Levi shall bring the offering of the corn, of the new wine, and the oil, unto the chambers, where are the vessels of the sanctuary, and the priests that minister, and the porters, and the singers: and we will not forsake the house of our God.
Bloodless sacrifices are acceptable but any sacrifice from a wicked man with a wicked heart is unacceptable and an abomination to God.
1Sa 15:22 – Behold, to obey is better than sacrifice, and to hearken than the fat of rams.
If Cain had been a righteous man, instead of a wicked man, who did well, his offering would have been accepted.
Gen 4:7 – If thou doest well, shalt thou not be accepted?
Biblical Truth Resources 
The first theory I heard was that because Cains offering had no blood it was not acceptable. But there were grain offerings in the Bible.