Genesis 4:2-5 - What was Wrong with Cain's Sacrifice?

Now Abel was a keeper of sheep, and Cain a worker of the ground. 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. So Cain was very angry, and his face fell. –Genesis 4:2-5

Why exactly Cain'a offering was rejected by God is considered an open question, but it is clearly about worship. Therefore let's go back to before that to see how God instituted offering in worship. At first man walked with God and heard his word directly. Worship was a part of everyday life. They were continually clothed with God's righteousness.

And the man and his wife were both naked and were not ashamed. -Genesis 2:25

However, after the fall, they were no longer clothed with God's glory, so they tried to cover themselves with plants, which are perishable.

Then the eyes of both were opened, and they knew that they were naked. And they sewed fig leaves together and made themselves loincloths. –Genesis 3:7

God showed that blood sacrifices are needed to cover up our unrighteousness by killing animals in the garden for clothing.

And the Lord God made for Adam and for his wife garments of skins and clothed them. –Genesis 3:21

The Bible does not actually say specifically whether such sacrifices had been commanded by God, or whether the practice arose merely as a spontaneous expression of thanksgiving and worship. If it was the latter, however, it is difficult to understand why God would not have been as pleased with an offering of Cain’s fruits as with an offering of Abel’s slain lamb. It seems more likely that God did give instructions, and that Cain had disobeyed. The entire occurrence can only be really understood in the context of an original revelation by God regarding the necessity of substitutionary sacrifice as a prerequisite to approaching God. Such revelation was most likely given at the time God provided coats of skins for Adam and Eve, and then banished them from his presence, providing, however, a specific means by which they could still commune with Him at certain times, on the basis of a similar sacrifice.

If this is the case, then Cain made up his own worship by offering his fruit. This speaks against making up our own worship, but also is speaking against "offering your own fruit." When we try to offer Him the fruit of our labor and he also rejects it.

What are the alternatives as to why God rejected the sacrifice? Poor quality? Nowhere does it say that. He gave what he had.

The heart of the person making it? The Bible does not say that. Cain brought an offering, in the course of time, of his own volition. Also, he was surprised, angry, sad and ashamed that it was not accepted, so it does not appear that he brought it just to fulfill a duty, but because he wanted to.

Hebrews tells us it was Able's sacrifice itself that was more acceptable to God. Cain did bring the Lord an offering as was mad when it was not accepted, so he must have had good intentions. Also, not just any offering, but a sacrifice, since it was part of his crop, which was his livelihood.

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. –Hebrews 11:4

Just before that, the author of Hebrews talks about sacrifices and how only blood atones for sin. 

Indeed, under the law almost everything is purified with blood, and without the shedding of blood there is no forgiveness of sins. –Hebrews 9:22

The Hebrew word used for “offering” in Genesis, minchah, can also be translated “sacrifice” or “meat offering” according to Brown–Driver–Briggs, and is used elsewhere to refer to meat offerings [e.g., Leviticus 7:37; 14:21; 23:18]).