God's division of the 10 Commandments was marked as "Duties to God," and "Duties to Man" (Mark 12:28-31). This in Hebrews 8 is from Jeremiah 31:31ff and is quoted by the author. God long ago had prophesied in this very place that it would be done away with & replaced with a new covenant. To be sure, the author could have quoted some very convincing and powerful words of Christ and his apostles as sufficient authority for hailing the old covenant as obsolete and abolished; but it should be kept in mind that he was addressing a group of people who had a strong emotional tie with the Old Testament, and was therefore better procedure on his part to prove his proposition from Old Testament. For proper identification of the "covenant" Jeremiah had in mind, the one to be abrogated, Hebrews 8:1-7. Gives Two basic reasons why the old covenant was abolished & nailed to the Cross. Galatians 3:1ff teaches that "... the covenant, that was confirmed before of God in Christ, the law, which was four hundred and thirty years after, cannot disannul, that it should make the promise of none effect."