He took us through the whole Bible, from Genesis to Revelation, telling us of animal sacrifices, and finally, the ultimate sacrifice. Adam and Eve reaped the benefits of the first sacrifice. They sinned and suddenly knew that they were naked. God provided clothes for them of an animal's skin, the very first shedding of blood that atoned for the sin that opened their eyes to their nakedness.
A few chapters later, God provided a substitute for another man. Abraham was willing to offer up Isaac, but did he know God would provide a substitute for him (Gen 22:4-8)? He provided a substitute in the same city, on the same hill on which He would later provide a lasting substitute for sin.
One of a few more stops in the Old Testament tells us of Noah making sacrifices for the spared lives of him and his party as soon as the boat came up on shore. In Exodus, the children of Israel used the blood of a male, unblemished lamb as a sign to escape death. "When I see the blood, I will pass over you." Job made animal sacrifices for the sins of his children just in case they had.
Blood makes us cringe, and it's for good reason. Leviticus says the life is in the blood. A friend of mine once told me, as a nurse, "When you see blood coming out of your patient, you recognize that their life is going out from them. You stop that blood in any way that you can." Just so, the animal's life was going out, and God made it clear that the Israelites were to witness it. They were to experience the horror. Their sins, according to justice, required life, the animal's life for theirs.
Isaiah wrote of the same blood in chapter 53, but a different type of sacrifice. This sacrifice was God's firstborn, and after his sacrifice, no other atonement would need to be made.
Romans 3:25-26 "God presented him as a sacrifice of atonement, through faith in his blood. He did this to demonstrate his justice, because in his forbearance he had left the sins committed beforehand unpunished— he did it to demonstrate his justice at the present time, so as to be just and the one who justifies those who have faith in Jesus." The entire New Testament is based on the atonement that Jesus made for sin once for all.
Finally, in Revelation, we see that we'll wash our robes so we can take part in the Marriage Supper of the Lamb. What will we wash our robes in? "...washed in the blood of the lamb..."
The story of atonement is found throughout the entire Bible, and it's a bloody affair. It's part of what makes it grip us. It's part of why it changes our whole lives when it does.