Summary: The lives of Cain and Abel teach us some important lessons about ourselves and our spiritual lives.
The root of sin is disbelief and disobedience to God –God said, “Don’t eat of the tree of the knowledge of good and evil.” The serpent talked Eve into doubting God – and the result of her disbelief was disobedience.
Observation for the Christian home: Children will learn from their parents.
Abel managed to learn about what sacrifices to make to God, and how to worship.
Cain learned the lessons of disbelief and disobedience.
I won’t hazard a guess at which boy learned from which parent; you be the judge of that!
Perhaps Eve expected Cain, her first son, to be the fulfillment of God’s promise in 3:15.
Perhaps he was coddled because of that assumption. Who knows? But Cain was not the one. Far from being a Savior, he was a murderer. Note: We can’t determine how God will carry out his blessings. He doesn’t need our help (Sarah and her handmaiden, etc.)
The fruit of sin, in this instance, was apostasy (falling away from God, which we will see Cain did by the way he worshipped – or, rather, failed to worship, God) and murder.
1. God wants our best
v. 3 “In the process of time” = “at the end of days,” which would mean on the Sabbath Day, the Day God had rested. There is a God-appointed day for worship.
Cain and Abel both brought of their first fruits.
Why was Cain’ not accepted? It was his best; but it was not God’s best.
God not only appoints a day of worship, but a way of worship.
We can’t just worship any old way we want to , and offer whatever sacrifice is pleasing to us. In order for our worship to be acceptable, it must be pleasing to God.
God has revealed, in His Word, the way He wants the New Testament Church to worship.
We know that God had revealed to Cain an Abel the way He desired to be worshipped, as well. See Hebrews 11:4 (quickview)  “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.” How could Abel make an offering by faith? Romans 10:17 (quickview) .
Cain brought his best, but it wasn’t God’s best. It wasn’t second-rate. It wasn’t defective. It would have won a blue-ribbon at the county fair. He brought the best of his beautiful, delicious fruit, and he brought it as an offering to the Lord. BUT IT WAS NOT THE OFFERING, IT WAS NOT THE KIND OF SACRIFICE THAT THE LORD REQUIRED. It was a bloodless offering.
In the OT, to worship God you brought a sacrifice of blood – a sacrifice which pointed to the Redeemer who was coming into the world, who would crush the serpents head. You didn’t come by bringing the works of your own hands, as if somehow you could earn your own salvation. You couldn’t then, and you can’t now. Without the shedding of blood there is no remission of sins (Hebrews 9:22 (quickview) ).
“There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it.” (1 Corinthians 10:13 (quickview) )