1) Two classes of worshipers emerged after Adam and Eve sinned - those who worshipped God according to His standards, and those who invented their own methods of worship.
2) Cain and Abel represented these two classes - Cain's offering of works was rejected while Abel's offering of faith through sacrifice was accepted.
3) Over time the lines between the two classes blurred as more people turned away from worshipping God, though some like Noah remained faithful. Abraham was also called to be the father of God's chosen people who would worship Him.