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Cain was a faithless man who lived his life in a way which displeased God. When he brought his offering to God it was not accepted.
Genesis 4:3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto 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: 5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell. 6 And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen? 7 If thou doest well, shalt thou not be accepted?
‘doest well’ means ‘do good’ or ‘that which is right’. The NIV translates it:
NIV Genesis 4:6 Then the LORD said to Cain, "Why are you angry? Why is your face downcast? 7 If you do what is right, will you not be accepted?
Some suppose that Cain should have offered an animal. The scriptures indicate that Cain’s whole attitude and way of life was evil. The New Testament writers’ inspired comments are as follows:
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 works were evil, and his brother’s righteous.
It may reasonably be supposed that his motive for killing Abel was envy. This was why the rulers of the Jews killed Christ and they were of their spiritual father who was a murderer.
I hope you have found this helpful.
May God bless you,