He did not enter by means of the blood of goats and calves; but he entered the Most Holy Place once for all by his own blood, having obtained eternal redemption.

Blood of Christ

The idea of animal sacrifice seems ancient and even barbaric to most people today.  The idea of killing something to appease a god is something that we think of happening in pagan tribes thousands of years ago.  But blood is central to Christianity as well.

Blood sacrifice was instituted at the very first sin.  When Adam and Eve sinned and realized that they were naked, God sacrificed an animal and covered their nakedness with the skin.  The leaves that they hid behind were not adequate.

The problem with animal sacrifice was that it was only a covering.  The sins were covered over but not actually erased.  Old Testament saints were still saved by virtue of following the sacrifices because of their faith however.

Every sacrifice that was made - year after year - pointed toward Jesus.  Undoubtedly, the Israelites didn't understand all of it but they did what God asked regardless. 

On the Day of Atonement, once a year, the high priest could enter the Most Holy Place and make a sacrifice for the nation of Israel.  When Jesus died and His blood was shed, the curtain that separated the Most Holy Place was torn in two.  Jesus' blood literally tore down the barrier between God and man. 

Jesus was the once for all sacrifice.  No more sacrifice is needed now.  Just like the Old Testament saints, we are saved by faith in Jesus.  We have the benefit of a much better understanding than they did however.