This material discusses Cain from the perspective of the "New Testament" teachings as well as the Old Testament. Therefore, there are statements in this material which would be rejected by those who believe in the legitimacy of Genesis but who deny the legitimacy of the New Testament.
“And Jehovah respected Abel and his offering, but He did not respect Cain and his offering.” (See Genesis 4:4-5)
Why did God reject Cain's sacrifice?
"And Jehovah said to Cain, "Why are you furious? and why is your countenance fallen? If you do well, shall it not be lifted up? and if you do not well, sin crouches at the door; and to you shall be its desire; but you should rule over it." (Genesis 4:6-7)
There was something about what Cain did that was not done well. But what was it?
The epistle to the Hebrews says that Abel offered his sacrifice by faith. (Hebrews 11:4)
Now some have jumped to the conclusion that because Hebrews 11:4 uses the word “faith” that Cain had less conviction than Abel, but that does not fit the context of Hebrews 11. The subject of Hebrews 11 is not the conviction of believers but the obedience of believers. Let’s look at some examples.
v. 7 Noah obeyed and built an ark
v. 8 Abram obeyed and left his home
v. 17 Abraham obeyed again and offered up his son.
v. 28 Moses obeyed and kept the Passover
v. 29 The Israelites obeyed and crossed the Red Sea
v. 30 Joshua and Israel obeyed and compassed the walls seven days
In Romans 10:17, Paul states that Faith originates with hearkening to that which God has said.
"So faith of hearing, and hearing by the word of God."
We can compare that to the previous list.
Noah did not come up with the idea of an Ark, God chose an ark
Abram did not choose which land he would go to, God chose where Abram would go.
Moses did not come up with the idea of blood on the doorpost and on the lintel, God chose that act of faith
Joshua did not come up with the idea of marching around Jericho, God chose that means of putting trust in God's promise.
In every story of faith in the Bible, it was God who made the choices and man demonstrated his reliance on the trustworthiness of God's promise by doing that thing which God had chosen.
And the thing which God chose was itself impotent. When God commanded Moses to build a brass serpent, the brass serpent had no ability to render the venom harmless. When God commanded Joshua to march around Jericho, it was not the vibrations of Israel's footsteps nor the sound of the final shout which caused the walls to fall. God always chose things that would demonstrate that what man did was itself impotent.
"And Jehovah said to Gideon, The people that are with you are too many for Me to give the Midianites into their hand, lest Israel boast themselves against me, saying, Mine own hand has delivered me."
This point is fundamental to our understanding of God's choices. God has always deliberately chosen things which man does not admire nor respect and which man considers insignificant and impotent. The religious world in general chooses things which men admire and respect and which men consider effective and important. Thus most people who claim to believe in Jesus demonstrate a lack of faith in Jesus.
"But God chose the foolish things of the world, that he might put to shame them that are wise; and God chose the weak things of the world, that he might put to shame the things that are strong; and the ignoble things of the world, and the things that are despised, God chose and the things that are nothing, that he might bring to nothing the things that are: that no flesh should glory before God."
Cain believed that God exists but just like those who today follow after their own choices (lusts) while claiming to be Christians, he tried to offer a sacrifice to God that was not founded on submission to God's word and trust in God's eternal reliability.
"Not as Cain <who> was of the evil one, and murdered his brother. And why did he murder him? Because his works were evil, and his brother’s righteous." 1 John 3:12
If you claim to believe in Jesus and deny Him the right to choose, if you are not content to abide solely in those things which the Bible states that God has chosen, then your belief is not faith.
"Even the demons believe and tremble."