S: Genesis 4:3-7 At the designated time Cain brought some of the fruit of the ground for an offering to the Lord. But Abel brought some of the firstborn of his flock – even the fattest of them. And the Lord was pleased with Abel and his offering, but with Cain and his offering he was not pleased. So Cain became very angry, and his expression was downcast. Then the Lord said to Cain, “Why are you angry, and why is your expression downcast? Is it not true that if you do what is right, you will be fine? But if you do not do what is right, sin is crouching at the door. It desires to dominate you, but you must subdue it.”
O: Some people think the reason Cain's offering didn't please the Lord was because he offered the results of his agriculture, rather than a blood sacrifice, and this is unfair because Cain was a farmer while Abel was a shepherd.
However, this was before the Mosaic law was instituted, and even in the Mosaic law, agricultural sacrifices were part of the offerings. And Hebrews 11:4 seems to suggest that the reason Abel's sacrifice was acceptable to the Lord was "by faith".
Anyway, regardless of why Cain's sacrifice didn't please the Lord like Abel's did, the point of this story was that Cain became jealous of Abel and angry with him, and God warned him that he had no basis for this animosity towards Abel -- it wasn't Abel's fault, it was Cain's own fault.
Furthermore, Cain's nurturing of this grudge meant that sin was crouching at his door. God urged Cain to subdue it, but as we know, Cain instead continued nurturing his grudge until it ended in murder -- fratricide.
A: We find ourselves annoyed with someone, Christ calls us to quickly forgive -- e.g. Matthew 5:23-24, Luke 11:4, Matthew 6:14-15.
P: Father, help us not to hold grudges. May we forgive as You have forgiven us. In Jesus' name, amen.
Note: this is using the SOAP method. For more information, see this page (not written by me.)