A lot of people complain about the quality of many of the priests we have today (pray more) but God must have wondered what He had started when He looked at the quality of the first priests! Adam blew it and gets shut out of the Garden and then we get Cain and Abel.
As first born son Cain would have had a special role as priest in the family-but Cain had ideas of his own it seems. We get the impression from Scripture that Eve thought Cain was the promised one and spoiled him a bit. Abel was less loved it appears. Then they grow to be priests and come to make their sacrifices. According to Jewish tradition these sacrifices were for their brides as they entered married life. Again we see the role of priest is to sacrifice to God for his bride.
In the offered sacrifices we see the symbols of bread and the lamb; Abel offers the unblemished lamb and Cain just offers some grain -but not the best. St Francis of Assisi made sure that the churches he had anything to do with had the most beautiful adornment and chalices of gold and precious stone. Only the best is good enough for God he reminded the people. Mother Angelica followed the same principle when she built her beautiful church with all the best materials she could find.
Cain didn’t feel the need to give God the best he could which I believe would not only have been an insult to God, but to the bride Cain was supposed to be sacrificing for. God didn’t accept the sacrifice. God did however accept the sacrifice of Abel who understood that both God and his bride deserved the very best he could offer.
Cain was jolly annoyed by all this. He got a bit stroppy and in a fit of envy went and murdered his brother. Now, Scott Hahn reckons there was an element of human sacrifice here and it certainly fits neatly. Cain says spitefully, “So You want the best, I’ll give You the best…” and kills Abel. (The fact that Cain apparently was considered the ‘best’ until this point isn’t lost here either). Cain, having offered the sacrifice takes his wife and has to make a run for it after God tells him he is in big trouble.
God then gives Adam and Eve a new ‘first born’ son Seth and from him the priesthood of father to son continues. God does not give up on the priesthood even though it became very badly corrupt very early on.