And Abel also brought of the firstborn of his flock and of their fat portions. And the Lord had regard for Abel and his offering, but for Cain and his offering he had no regard. Genesis 4:4-5a
Genesis 4 contains the first example of worship in the Bible. The passage is not easy. Why did God approve Abel's worship but not Cain's?
The text does not explicitly say why. Some readers suggest it was because Abel had a blood sacrifice and Cain did not. But there is nothing inherently wrong with bloodless sacrifices. In fact, we see examples of bloodless sacrifices in the Mosaic Law. It is true that sin offerings required the shedding of blood but we don't know this was a sin offering.
A better explanation is that Abel's heart was right before God and Cain's was not. That is, the key difference is not the type of sacrifice but the hearts of the worshipers. Several things hint at this explanation.
Cain brings "of the fruit" whereas Abel brings his very best, his "firstborn" and "their fat portions." Cain simply discharges his duty whereas Abel gives his very best.
Also, the text does not say that God looked with favor on Abel's offering but on Abel and his offering. (Admittedly, a subtle point!) Furthermore, Cain's attitude is revealed by his reaction to God's disapproval: not humility and contrition but jealousy and anger. The response revealed Cain's heart, which God of course already knew.
Finally, Hebrews 11:4 tells us that "By faith Abel offered to God a more acceptable sacrifice than Cain." His sacrifice was not inherently better but it was better because it came from the heart of faith.
In conclusion, we see the worship that pleases God is worship that comes from a worshiping heart, worship that brings our very best to God, worship that comes from a genuine heart to please God and trust God.
God does not want our religious duty. He does not want our religious ritual. He wants our hearts. He is seeking worshipers, worshipers who cheerfully, gratefully, wholeheartedly, bring their best to God.
Does God see in you a heart of worship?