Why is blood required to be forgiven of sins?

This idea goes all the way back to the Old Testament. It was the blood of year-old lambs without defect, sprinkled on the doorposts of Hebrew homes, that caused the angel of death to “pass over” the Israelite homes during the last plague just prior to their exit from Egypt. Later on, in the book of Leviticus, God expanded further of His view on the significance of blood: For the life of a creature is in the blood, and I have given it to you to make atonement for yourselves on the altar; it is the blood that makes atonement for one’s life—God speaking, Leviticus 17:11. Simply put, blood represents life. In the Old Testament sacrificial system, God wanted the Israelites to know that, when blood was sprinkled on the altar or, once a year, on the ark of the covenant, among the many statements being made was that something had died to provide that blood, giving an substitutionary atonement that they might live and that their sins were forgiven. In the Old Testament, the person bringing the sacrifice would place his hand on the animal and, as it was being killed, he would feel the life leave the animal. The sacrificial system gave the cost of sin a very intimate feel—life for life. The idea was that, on the one hand, the blood represented the sinner’s life, infected by his sin and headed for death. On the other hand, the blood represented the innocent life of the animal that was sacrificed in place of the guilty person making the offering. The death of the animal (of which the blood was proof) fulfilled the penalty of death; therefore, God granted forgiveness to the sinner. In fact, the Hebrew word for atonement is “kaphar,” which means “to cover.” The blood of animals was seen as means of covering up sin so that God could no longer see it. And, in the death of an innocent animal, that innocence is passed on to the sinner. But it is God Who forgives based on the faith of the person doing the sacrificing. And as the writer of the New Testament book of Hebrews pointed out, In fact, the law requires that nearly everything be cleansed with blood, and without the shedding of blood there is no forgiveness—Hebrews 9:22. Of course, the great substitutionary shedding of blood for us came at the expense of Jesus Christ; as Isaiah pointed out, But he was pierced for our transgressions, he was crushed for our iniquities; the punishment that brought us peace was upon him, and by his wounds we are healed—Isaiah 53:5. Animals could not always be a substitute for mankind; eventually, humanity would need one of their own. God always wanted mankind to know that sin comes with a price; in the Old Testament, animals paid the price and, in the New Testament, God’s own Son did that. As the writer of Hebrews said so well, 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—Hebrews 9:12.