Leviticus 17:10
And whatsoever man there be of the house of Israel, or of the strangers that sojourn among you, that eateth any manner of blood; I will even set my face against that soul that eateth blood, and will cut him off from among his people. And whatsoever man there be of the house of Israel, or of the strangers that sojourn among you, that eateth any manner of blood; I will even set my face against that soul that eateth blood, and will cut him off from among his people.
The command forbids eating blood in any form, including flesh eaten with its blood still in it as in the case of animals that have been strangled. It was laid as a perpetual statute in all dwellings and for all generations, binding both the house of Israel and the strangers sojourning among them, and it covered not only beasts offered in sacrifice but all cattle used for food. The flesh of a permitted animal could be used for food only after its blood had been carefully drained from the body. Whoever ate blood brought God Himself into opposition, for He would set His face against that soul and cut that person off from among the people.
The reason given is that blood was regarded as the life, for the life of the flesh is in the blood, and the shedding of blood exists only in consequence of sin. Blood is sacred because God gave it upon the altar to make an atonement for souls, for it is the blood that makes an atonement for the soul. Atonement comes only through the shedding of the blood of the Son of God, symbolized by the blood of the slain victim, and the blood used to cleanse the sanctuary from the sins of the people typifies the blood of Christ which alone can cleanse from sin. By forbidding its common use as food the law preserves a clear distinction between the sacred and the common and guards against the idolatrous practice of drinking sacrificial blood, which produces confused ideas. It was also given to preserve health and strength, since meat with the blood retained is not healthful, and the Jews treated it as a divine instruction for health.
As blood is the life of the human body, circulating from the heart to nourish bone and sinew and muscle and keep the whole body healthful, so Christ is the life of the soul. Spiritual life and health are maintained only by vital connection and personal contact with Him through faith in the blood of Christ, whose efficacy supplies every need of the soul. A soul without Christ is like a body without blood, dead even if it keeps up an appearance of spiritual life and performs religious ceremonies like a machine, and pretentious Christians who do not live His character and are not in vital connection with Him He will reject.
The force of the law is seen after Israel's hard day's battle with the Philistines, when the soldiers were faint from labor and from Saul's rash vow barring them from eating until evening. Once the time of the vow expired their faintness drove them to violate this command by eating the animals taken in battle without draining the blood, which was transgression of the commandment of the Lord. The law had become a fixed Jewish custom to drain the blood and to treat eating blood as sinful. Jews were therefore shocked and outraged by Gentiles who caught and drank the blood flowing from sacrifices or used it in preparing food, and Jew and Gentile could not eat at the same table while those habits continued.
That background explains why the Jews were horrified by Jesus' words about eating His flesh and drinking His blood. They appealed to this ritual prohibition and took His words in the most literal sense as calling for a forbidden act.