One question that arises out of the Cain and Abel story that is not explicitly addressed in the text is why God rejected Cain's offering but accepted Abel's. I personally think the reason is inferred in the text; whereas Abel brought from his firstlings, Cain simply brought sacrifices--he did not offer God the best he had to offer.
Apparently, I am not alone in this view. Below are some various ancient interpretive takes.
Philo, The Sacrifices of Cain and Abel, 52: “[Cain’s offering] was ‘of the fruit’ but not ‘of the first fruit.'"
Genesis Rabbah 22:5: “[Cain’s offering] was from the refuse. The matter may be compared to the case of a wicked sharecropper who ate from the first fruits of the figs and handed over to the king the late figs.
Ephraem, Commentary on Gen. 3:2: “Abel chose and brought for sacrifice from the firstborn and the fattest, but Cain brought [merely] the fruits he found at the time . . . [God] chose not to accept his sacrifice from him in order to teach him how it was to be offered up… [Cain] did not bring these on the day of the first fruit offering, but brought the fruit of his land.”
Heb. 11:4: “By faith Abel offered to God a more acceptable sacrifice than Cain.”
1 John 3:11-13: 11 For this is the message which you have heard from the beginning, that we should love one another, 12 and not be like Cain who was of the evil one and murdered his brother. And why did he murder him? Because his own deeds were evil and his brother's righteous.