i am not trained in theology, nor do i possess the gift of gab.
i can only relate in simple terms, what is revealed by the Holy Spirit.
all this aside, in the case of Cain and Abel.
Gen 4:3 And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.
Gen 4:4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:
Gen 4:5 But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.
we are told in Heb.4:11 that Abel's sacrafice was accepeted because of his faith.
Heb 11:4 By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he was righteous, God testifying of his gifts: and by it he being dead yet speaketh.
if Able's faith was acceptiable then his sacrafice was acceptable.
Cain on the other hand, and his offering, were not accepted. why ? because of lack of faith,jealousness,anger,covetousness.(fruits of the flesh)
we are not sure what Gods commandments were when making an offering at this time,but we know some system was set up because the brothers were offering.
also lets look at Gen.3:7
Gen 3:7 And the eyes of them both were opened, and they knew that they were naked; and they sewed fig leaves together, and made themselves aprons.
evidently the fig leaves were not sufficient to cover their sin and shame. because in Gen.3:21
Gen 3:21 Unto Adam also and to his wife did the LORD God make coats of skins, and clothed them.
this showes that it took more than , fig leaves ,fruits,and veg. to cover sin. it took the inocent blood to atone for the sinful nature they had inherited.
Abel, by faith offered a blood offering,Cain by his fleshly reasoning offered fruits,