The heart of the matter is the matter of the heart!

People worship God in many different ways, and for some, their worship is
not according to the truth of God's Word. Cain and Abel came to worship
God, and their ways of approaching God are a contrast as far removed as the
east is from the west. *Gen 4:2...Now Abel kept the flocks [of sheep and
goats], but Cain cultivated the ground. 3 And in the course of time Cain
brought to the Lord an offering of the fruit of the ground. 4 But Abel
brought [an offering of] the [finest] firstborn of his flock and the fat
portions. And the Lord had respect (regard) for Abel and for his offering;
5 but for Cain and his offering He had no respect.* One decided he would
try to “work” his way into God’s good graces. Cain, a farmer, simply gave
an offering, worshiped the LORD his way by bringing vegetables and fruit of
the ground to offer the LORD, he thought, surely God would accept any
well-intentioned, beautiful offering! Abel, a shepherd, took a little
animal—possibly a spotless lamb—and with shed blood he worshiped the LORD.
This passage definitely focuses on the quality and heart behind the
offering. Abel gave the firstlings of his flock and of their fat portions.
It wasn't just any offering— it was the best he had. Cain did give an
offering, but he did not give it in the right manner. He was a hypocrite.
He simply gave so that he might appear to be righteous, but God saw through
Cain's false motives. Cain represents salvation by works. He tries to be
right with God by what he does. Our fallen human nature thinks that if we
bring the fruit of our labor, toiling away, and offer it to God, He is
going to accept it…surely He will! Remember, the heart of the matter is the
matter of the heart. And God knows what is in our hearts.

Douglas