It was the will of the Lord, from the beginning of time, that He should be worshiped. When Adam and Eve disobeyed and ate of the fruit of the forbidden tree they were disobeying, thus they were failing to revere and honor God. (Genesis 3:1-13) When their sons, Cain and Able were of a working age, Cain became a farmer and Able tended to flocks, as early as this first family we see where God prescribed the way he wanted his creation to worship Him.  He told them he wanted a sacrifice to be offered (Genesis 4:3-5).