What does Hebrews 9:15 mean?
The writer of Hebrews has been very careful to point out that God's plan was always to provide a "new covenant" for His people. Melchizedek's priesthood is one such proof (Hebrews 7:11). God's own decree to Jeremiah is another (Hebrews 8:8–9). The flaws and drawbacks of the old covenant were not an accident; rather, those were meant, by God, to help us understand the need for the new covenant (Hebrews 8:7). In recent verses, the writer has indicated that animal sacrifices have value, but that value is limited. They can provide ritual purity, but cannot change us on the inside. Those sacrifices cannot really remove the penalty of sin. The sacrifice of Christ, on the other hand, can accomplish what those other sacrifices cannot.This makes the new covenant the "promised eternal inheritance" which God has always intended for mankind. The original audience of this letter was persecuted Jewish Christians. This reference to inheritance reminds them that following Christ is not an abandonment of their Jewish roots. Instead, it is a fulfillment of that heritage. The sacrifice of Christ, in fact, is the event which actually provides for forgiveness of all sins—including those which occurred during the ages before His birth.
According to this passage, animal sacrifice cannot fully atone for human sin. This is one reason that the priests of the old covenant had to constantly offer sacrifices. And, a sinful human priest has to offer sacrifice for his own sin. Christ, in contrast, offers a sacrifice which is completely human, completely sinless, and completely effective in what it accomplishes.
This is one of three places in the book of Hebrews where Jesus is described using the term mediator (Hebrews 8:6; 12:24). A mediator resolves a conflict between two sides.