1 Now Adam knew Eve his wife, and she conceived and bore Cain, and said, “I have acquired a man from the Lord.”
2 Then she bore again, this time his brother Abel. Now Abel was a keeper of sheep, but Cain was a tiller of the ground.
The following paragraph by Morris is conjecture, but I think it makes sense enough to be a possibility.
It seems reasonable to infer that, after the expulsion from Eden, God had made gracious provision to continue to commune with man, even though now “at a distance,” on the basis of His promise of a coming Redeemer, whose shed blood would be the price of redemption. He had shown Adam and Eve that an “atonement” required the shedding of innocent blood to provide a “covering” for the guilty. Probably at an appointed time and place, men were able to meet God, first being careful to approach Him by means of a proper offering, especially marked by the principle of substitution—the innocent for the guilty. — Morris, page 133.
This is the first use of the familiar Biblical euphemism for marital intercourse; “Adam knew his wife.” Such an expression uniquely emphasizes both the full harmony and understanding of man and wife (one flesh) and also an ideal awareness of God’s primeval purpose as implemented through the human capacity for sexual love and reproduction.
The name Cain means “gotten” and is obviously derived from Eve’s exclamation of joyful acquisition. … Eve not only was thankful for a child, but also that the Lord had enabled her to begat a man. This seems to be a further expression of faith that her babe would grow to manhood. It is possible that she hoped this might be the promised Deliverer, even though he was not in a specific biological sense a “seed of the woman.” As a matter of fact, he “was of that wicked one” (1 John 3:12), and thus was the first in the log line of the Serpent’s seed.
Cain’s younger brother, Abel, was truly in the household of faith, however, He is the very first mentioned in the long line of men of faith recorded in Hebrews 11 (v.4). He is called “righteous” and a prophet (Matthew 23:35; Luke 11:50-51). … As a prophet, he must also have received God’s Word by divine revelation and preached it by divine enablement. But Cain refused it and disobeyed.
the name of Abel means “vapor” or “vanity,” and suggests that, by the time of Abel’s birth, Eve had become thoroughly impressed with the impact of God’s curse on the world. God had indeed made the creation “subject to vanity” (Romans 8:20).
As the boys grew, Cain became a farmer and Abel a shepherd. Both were honorable occupations, Cain’s fruits provided food and Abel’s sheep providing clothing for the family. In addition, it is probable that the sheep were to be used for sacrifice. … Man was not authorized until after the Flood to use animals for food (Genesis 1:29; 2:16; 3:19; 9:3). —Morris, pages 134-135.
In the persons of Cain and Abel, the first examples of a religious man of the world and of a genuine man of faith. Born, as they were, outside of Eden, and being the sons of fallen Adam, they could have nothing, naturally, to distinguish them one from the other. They were both sinners—both had a fallen nature—neither was innocent. …
What, therefore, made the vast difference? The answer is as simple as the gospel of the grace of God can make it. The difference was not in themselves, in their nature or their circumstances; it lay entirely in their sacrifices. This makes the matter most simple for any truly convicted sinner—for any one who truly feels that he not only partakes of a fallen nature, but is himself, also, a sinner. The history of Abel opens, to such an one, the only true ground of his approach to, his standing before, and his relationship with, God. It teaches him, distinctly, that he cannot come to God on the ground of anything in, of, or pertaining to, nature; and he must seek, outside himself, and in the person and work of another, the true and everlasting basis of his connection with the holy, the just and only true God. The eleventh chapter of Hebrews sets the whole subject before us in the most distinct and comprehensive way, “By faith Abel offered to God a more excellent sacrifice than Cain, through which he obtained witness that he was righteous, God testifying of his gifts; and through it he being dead still speaks” (Hebrews 11:4).
Thus the first dispensation ended in failure, yielding as its result a mournful proof that man is a being too weak to retain his innocence even in the most favourable circumstances. it now remained to be seen whether after the experience of the fall, after tasting the bitter consequences of sin, he could recover his position and become again obedient and holy. Of this God made trial in several ways.
First, in what we may term the age of freedom, during the lapse of which He left Adam and his descendants almost entirely to their own devices. Marriage had indeed been instituted: and they were instructed to approach Bod by means of typical sacrifices, and commanded to toil for their bread by tilling the earth. But beyond this God would neither Himself issue laws nor suffer men to do so. The sword of the magistrate might not be used for the repression of crime: even the murderer should be unpunished, as we may see by the case of Cain. No government was permitted: every man should go in his own way, and do that which was right in his own eyes.
Thus the fitness of man for a condition of extreme liberty, and the worth of a trust in the innate justice supposed to lie at the bottom of the human heart, have been already tested by the great Creator. — Pember, pages 165-166.
Eve’s statement is to be translated, “I have acquired a male, the Lord.” In other words, Eve’s expectation regarding Cain, her first male child, is that he is none other than the promised “seed” of 3:15, who, as God incarnate, would restore humanity to their pre-Fall ideal by “crushing” Satan’s head and ending the reign of sin over Creation. — Wechsler, page 116.
In their desire to worship, therefore, the brothers quite naturally present to the Lord that which is theirs to give—namely, a fruit offering and a flock offering. The reason Cain’s offering is rejected is not because it was a non-animal offering (this distinction is only made later on in the Law of Moses, and even then fruit/grain offerings are specified as a legitimate type of offering; (cf. Leviticus 2:1.), but rather because Cain’s heart, or attitude, was not consistent with the act of worship. — Wechsler, page 116-117
I’ve heard Wechsler’s viewpoint before, but I don’t think I fully agree. Yes, the Mosaic law speaks of grain offerings, but as far as I can see from Leviticus 2, they are for memorials and worship, not for sin offerings. As Hebrews 9:22 says, “And according to the law almost all things are purified with blood, and without shedding of blood there is no remission.” There’s not blood in fruit. Yes, Cain’s attitude was wrong, but I don’t believe that was the only problem with his offering. That fact that Abel knew to kill a sheep to offer it to the Lord is evidence that God had revealed His will regarding blood sacrifices. Otherwise, there would have been no reason to kill the sheep.