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.

In what condition are all men?
Under the old covenant, what did the people promise?
What did Christ say would he the result if the days of persecution were not shortened?
Why will many not be prepared for this event?
What comforting promise has God made concerning the sleeping saints?
How long will they sleep there?
To whom must one belong in order to be Abraham's seed?

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