A pastor in Youtube, that I listen to, said he wasn't sure whether God had indicated to Cain and Abel - what kind of sacrifice they were to offer Him. But I think it's most likely that God explained to them ahead of time just what they should sacrifice. I think it's very reasonable that their first sacrifice was meant to be a lamb, or etc. to picture Jesus' sacrifice for sins. Taking care of sin must be done before thanking God for things.
It's true that Cain and Abel represent 2 kinds of spiritual conditions. Cain had the bad one, while Abel had the good one. Only Abel had the right attitude towards God.
Below is an explanation of the story, that I found online through the GotQuestions.org site:
Cain and Abel, the sons of Adam and Eve, "in the course of time" brought offerings to the Lord (Genesis 4:3). Without doubt, they were doing this because God had revealed it to them. Some question, “How were Cain and Abel supposed to know what to sacrifice?” The answer is that God must have instructed them. It is clear that the offering was to be a substitutionary atonement, because we read in Hebrews 11:4, "By faith Abel offered God a better sacrifice than Cain did." When Abel came for worship, it was by faith that he brought his offering, the "fat portions from some of the first-born of his flock" (Genesis 4:4). The Lord looked with favor on Abel and his offering, and it was accepted.
It's true that Cain and Abel represent 2 kinds of spiritual conditions. Cain had the bad one, while Abel had the good one. Only Abel had the right attitude towards God.
Below is an explanation of the story, that I found online through the GotQuestions.org site:
Cain and Abel, the sons of Adam and Eve, "in the course of time" brought offerings to the Lord (Genesis 4:3). Without doubt, they were doing this because God had revealed it to them. Some question, “How were Cain and Abel supposed to know what to sacrifice?” The answer is that God must have instructed them. It is clear that the offering was to be a substitutionary atonement, because we read in Hebrews 11:4, "By faith Abel offered God a better sacrifice than Cain did." When Abel came for worship, it was by faith that he brought his offering, the "fat portions from some of the first-born of his flock" (Genesis 4:4). The Lord looked with favor on Abel and his offering, and it was accepted.