Genesis 4:
2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
(Abel is a shepherd & Cain a farmer)
3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
(NOTE: Each brother brings an offering to the Lord)
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:
(NOTE: The fat thereof suggests Abel's offering was burned. In later time's the fat was claimed by the Lord as his own (Lev 7:4-5)
5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
(NOTE: The Lord no respect onto Cain's self works offering)
Hebrews 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
1 John 3:12 Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own (SELF) works were evil, and his brother's righteous.
Why was Abel's offering respected And Cain's rejected?
Abel's offering was brought thru faith & a firstling sacrifice.
Cain's offering was bloodless & brought via his own self works . MOO!