How early was this sacrificial system known?

Answer

"By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he
was righteous, God testifying of his gifts." Heb. 11: 4. See Gen. 4: 3-5; 8: 20.

How complete should this obedience be?
How was this fulfilled?
How is the law further described?
Is there any danger of God's chosen people forgetting Him?
What role do his agents assume?
Whom does God comfort?
What did it lead Moses to do?

Questions & Answers are from the book Bible Readings for the Home Circle