Re: Genesis 4:3-5.
In the course of time Cain brought some of the fruits of the soil as an offering to the LORD. But Abel brought fat portions from some of the firstborn of his flock. The LORD looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
Why did God not have respect for Cain’s offering ?
After the fall God taught the first family how to prepare and present acceptable offerings to Him. We know this from Genesis 4:6-7
Then the LORD said to Cain, “Why are you angry? Why is your face downcast? If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must master it.”
God told Cain that if he did what was right his offering would be accepted. This proves that Cain knew how to present an acceptable offering. Abel’s offering was the life of an innocent animal, an early model of the Messiah, and it was accepted. Cain’s was some of the produce he had grown, the works of his hands, and it was not accepted.
From this passage we see that right from the beginning man was taught that the shedding of innocent blood was the only remedy for sin, and we also see that right from the beginning man has tried to justify himself with the works of his own hands. As Cain soon proved by murdering his brother, this always leads to death.