Abel
And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground. And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. 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: But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell. And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen? 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. And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him. Genesis 4:2-8
For this is the message that ye heard from the beginning, that we should love one another. 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. 1 John 3:11-12
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 speaks. Hebrews 11:4
For thus it is written: “And it came to pass after certain days, that Cain brought of the fruits of the earth a sacrifice unto God; and Abel also brought of the firstlings of his sheep, and of the fat thereof. And God had respect to Abel and to his offerings, but Cain and his sacrifices He did not regard. And Cain was deeply grieved, and his countenance fell. And God said to Cain, Why are you grieved, and why is your countenance fallen? If you offer rightly, but dost not divide rightly, has you not sinned? Be at peace: your offering returns to yourself, and you shall again possess it. And Cain said to Abel his brother, Let us go into the field. And it came to pass, while they were in the field, that Cain rose up against Abel his brother, and slew him.” You see, brethren, how envy and jealousy led to the murder of a brother. Clement of Rome (A.D. 96) ch. 4
[These act] as Cain [did, who], when he was counseled by God to keep quiet, because he had not made an equitable division of that share to which his brother was entitled, but with envy and malice thought that he could domineer over him, not only did not acquiesce, but even added sin to sin, indicating his state of mind by his action. For what he had planned, that did he also put in practice: he tyrannized over and slew him; God subjecting the just to the unjust, that the former might be proved as the just one by the things which he suffered, and the latter detected as the unjust by those which he perpetrated. And he was not softened even by this, nor did he stop short with that evil deed; but being asked where his brother was, he said, "I know not; am I my brother's keeper?" extending and aggravating [his] wickedness by his answer. For if it is wicked to slay a brother, much worse is it thus insolently and irreverently to reply to the omniscient God as if he could battle Him. And for this he did himself bear a curse about with him, because he gratuitously brought an offering of sin, having had no reverence for God, nor being put to confusion by the act of fratricide. Irenaeus (A.D. 180) Ante-Nicene Fathers vol. 1 pg. 456-457
For at the beginning God had respect to the gifts of Abel, because he offered with single-mindedness and righteousness; but He had no respect unto the offering of Cain, because his heart was divided with envy and malice, which he cherished against his brother, as God says when reproving his hidden [thoughts], "Though you offer rightly, yet, if you dost not divide rightly, have you not sinned? Be at rest;" since God is not appeased by sacrifice. Irenaeus (A.D. 180) Ante-Nicene Fathers vol. 1 pg. 485
"Woe unto you, scribes and Pharisees, hypocrites, for you are like white sepulchers. For the sepulcher appears beautiful outside, but within it is full of dead men's bones, and all uncleanness; even so you also outwardly appear righteous unto men, but within you are full of wickedness and hypocrisy." For while they were thought to offer correctly so far as outward appearance went, they had in themselves jealousy like to Cain; therefore they slew the Just One, slighting the counsel of the Word, as did also Cain. For [God] said to him, "Be at rest;" but he did not assent. Now what else is it to "be at rest" than to forego purposed violence? Irenaeus (A.D. 180) Ante-Nicene Fathers vol. 1 pg. 485
Thus, also, when He gave the law of prayer, He added, saying, “And when you stand praying, forgive, if you have ought against any; that your Father also which is in heaven may forgive you your trespasses.” And He calls back from the altar one who comes to the sacrifice in strife, and bids him first agree with his brother, and then return with peace and offer his gift to God: for God had not respect unto Cain’s offerings; for he could not have God at peace with him, who through envious discord had not peace with his brother. Cyprian (A.D. 250) Ante-Nicene Fathers vol. 5 pg. 425