Ge:4:25: And Adam knew his wife again; and she bare a son, and called his name Seth: For God, said she, hath appointed me another seed instead of Abel, whom Cain slew. Ge:4:26: And to Seth, to him also there was born a son; and he called his name Enos: then began men to call upon the name of the LORD.
Adam was the first man. Cain should have been a first son committed to God but he was too sinful and obviously self centered. None of Cains sons were acceptable to God to act as the early type of priest ie: to offer acceptable sacrifices to God. Seth came and then as you see in verse 25 he was appointed to God. His firstborn son Enos could then offer acceptable sacrifice to God or that is to say function in office as a type of priest. Abel was doing this but Cain was jealous and murdered him.
(please don't confuse this with the later appointed office of priesthood established by God thru Moses and Arron)
This is my best thought on this but is still just my thought.
Also remember it had to be a blood sacrifice.
I really have to look at the big picture of the scriptures to understand some of these things and so my thoughts are not drawn from only this small part but come from a view of the stories and way things are explained later as well.