"Indeed, under the law almost everything is purified with blood, and without the shedding of blood there is no forgiveness of sin" Hebrews 9:22
The writer of Hebrews (we cannot be certain who it is humanly speaking, but ultimately it's the Holy Spirit) has been talking about how Jesus is the True High Priest, the Greater Moses, the One who made himself the sacrifice and made the blood of bulls, goats, and other animals unnecessary. But instead of going through those things, the focus for me is "without the shedding of blood there is no forgiveness of sin." Wow! There are many "theologians" and "scholars" (I use the term loosely here) who want to say that God could not have ordained Christ's suffering on a cross because that would be "Divine Child Abuse". Others paint God in a negative light as being a blood-thirsty God who desired animal sacrifice just like other false gods that were worshiped in the days of Israel long, long ago. But for reasons we're not going to get into here, those people and their claims are wrong and simply untrue.
The reason forgiveness cannot happen without the shedding of blood, as far as I see it, is two fold: first, it shows how seriously God takes sin, and second because God designed blood to be the very life source of every living person. I'm no anthropologist or biologist, but I do know that once blood stops flowing, a person stops living (good thing I went to school for that one!). God, in his perfect knowledge, wisdom, and passion for his glory, set up the sacrificial system for Israel in order to make them long for a sacrifice that would once and for all pay for sin. The untold number of animal sacrifices were not the point in and of themselves; they were signposts that without the shedding of blood there is no forgiveness of sin. God reminded Israel many times that "to obey is better than sacrifice" (I Samuel 15:22, for example), so we know God never saw animal sacrifice as something he enjoyed. It showed the serious nature of the cost of sin. Sin destroys life, disrupts and breaks our relationship with God and with others. Sin, both unseen in the heart and seen in actions and words, is an assault on the glory of God and the purpose of God's creation. Without someone paying for it, we are eternally in God's debt. Some may want to question why God would take this so seriously if he had perfect knowledge of all things that would happen in human history, but in the end I rest in knowing God is SO good that he requires payment with blood.
Here's the kicker: while God does require blood for the payment of sin, he also provided the blood for sin. The writer of Hebrews goes on just a few verses later to say "But as it is, he has appeared once for all at the end of the ages to put away sin by the sacrifice of himself" (Heb. 9:26). Christ came down into human history, sent by the Father on a mission to die for his people, shedding his blood to bring many into glory. He died so that we might have life. He absorbed the debt, he paid our debt (not just canceling it, he paid for it in full), and he went even further than that. For if Christ had just paid our debt bringing our "balance" if you will to "$0 owed", that would only be half of what we need. He has also given us his righteousness! He not only brings forgiveness but wholeness, newness of life, righteousness that only he has to give. We aren't brought back into God's house as servants, but as sons and daughters.
Sin must be paid for by blood. God takes sin so seriously that only the life, and ultimately the death, of the Son of God could make anyone right with him. We long for justice in this world, and rightly so. God is a just God - all sin will be paid for, either through the blood of his son or by the blood of transgressors for all eternity. Come to the cross and thank God for his provision of blood to pay for your sin and bring you to himself.