Hebrews 9:15 (KJV) And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance.
Jesus is the mediator of the New Covenant. By His death on the cross, He atoned for all sin. This was necessary in order for man to be saved. The death of Christ was for those before Christ, just as much as for those after the Lord left this earth. The sacrifice of Jesus guarantees the redemption of everyone who dies in the Faith. Let us remember that the blood of bulls and goats could not redeem man, but the blood of Jesus does.
(Read Acts 20:32)