You have probably heard it said that the reason Abel's offering was accepted was because he offered a spotless lamb (Christ), and the reason Cain's offering was rejected was because he offered the fruit from a cursed land. That is the most common interpretation provided, but is it correct?
No!
No, it is not correct.
This interpretation is imposed upon the biblical text. The people who do so are apparently illiterate, or too lazy to do their own work and merely rely on the erroneous work of others. If there was no mention of resting on the seventh day, or a requirement to do so, until Moses, then the same weight of argument applies here. You will find no mention of God requiring an offering of a spotless lamb until Moses. While it was a requirement under Moses and until Christ Jesus, you cannot read such requirements backwards. You cannot say that is what Abel did because Yahweh required it where there is zero mention of such a requirement. That is an argument from silence. The first mention of a lamb for an offering was in Genesis 22. Yahweh God would not require the offering of a spotless lamb until hundreds of years later.
So what does the text tell us?
"So it came about in the course of time that Cain brought an offering to the LORD of the fruit of the ground. Abel, on his part also brought of the firstlings of his flock and of their fat portions. And the LORD had regard for Abel and for his offering; but for Cain and for his offering He had no regard. So Cain became very angry and his countenance fell. Then the LORD said to Cain, "Why are you angry? And why has your countenance fallen? If you do well, will not your countenance be lifted up? And if you do not do well, sin is crouching at the door; and its desire is for you, but you must master it." Cain told Abel his brother. And it came about when they were in the field, that Cain rose up against Abel his brother and killed him." Genesis 4:2-8
"By faith Abel offered to God a better sacrifice than Cain, through which he obtained the testimony that he was righteous, God testifying about his gifts, and through faith, though he is dead, he still speaks." Hebrews 11:4
The reason Cain's offering was not accepted is a matter of the heart. Hebrews offers us some insight into this. The idea that his offering was rejected because it was fruit that came from a cursed land is nonsensical and ridiculous. Several of the offerings under the Law require fruit that comes from a cursed land. We are not told specifically why Cain's offering was not accepted. Some people claim he did not offer the first fruits (the best) of his crops. Perhaps. Hebrews does not say that Abel's offering was "better" because he offered a spotless lamb. Again, that is being imposed upon the text. Had Cain made his offering by faith, Yahweh would have accepted it. How do we know? Because He told Cain as much: "Why are you angry? And why has your countenance fallen? If you do well, will not your countenance be lifted up? And if you do not do well, sin is crouching at the door; and its desire is for you, but you must master it."
God may have slain a lamb in order to cloth Adam and Eve as a precursor to the requirement of the Law that would be fulfilled in Christ, but we have to be careful about reading into the text that which is not actually present. Too much of our theologies impose nonsensical beliefs upon the texts that simply are not there. If it is something you thought about and could be a possibility, then teach it as such, but . . . do not teach it dogmatically! If it is not explicitly spelled out in Scripture, then what you are offering is nothing more than conjecture. Ditch the use of proof text methodology, eisegesis, and Scripture twisting, and teach the Word of God in context based on the details that are actually present.
It is one thing to draw parallels and speculate, but it is quite another to teach those speculations dogmatically as if they are biblical truth. The Scriptures are inspired and God-breathed; not your opinions, speculations, conjectures, or fanciful imaginations. Read the text in context, taking into account chronology of events (What order are the book in history?), and then teach it accordingly.