Leviticus 17:12, commentary and pictures

No Israelite and no foreigner living among them was ever to eat blood, by a perpetual command governing all cattle used for food and not only beasts for sacrifice, as part of the ritual law for animal food in Canaan. Blood was sacred because it is life and because the blood of the slain victim symbolized the blood of the Son of God, through whose shedding alone there is atonement for sin. The blood that cleansed the sanctuary from the sins of the people typified the blood of Christ that alone cleanses from sin. The prohibition was to preserve clear and definite ideas between the sacred and the common, to impress that if there had been no sin there would have been no shedding of blood, and to guard against the idolatrous practice of eagerly drinking the blood flowing from victims, which produced confused ideas.

The law requires that blood be carefully drained from slaughtered animals before the meat may be eaten, so that an animal whose blood has not been drained cannot be used for food. This draining was for health and strength and for preparing healthful meat, and the related dietary restrictions in Canaan also forbade eating fat. Eating meat with its blood or even tasting blood is a transgression of God's commandment and is sinful.

After a day of hard fighting against the Philistines, the people grew faint from hunger under Saul's rash and selfish vow and order to eat nothing until evening. When the restriction ended they were so faint they fell on the spoil and devoured flesh with the blood, violating God's law. Saul's command led them to transgress.

The strict prohibition led the Jews to take Jesus' words about eating His flesh and drinking His blood literally as a sacrilegious command, failing to grasp His spiritual teaching. It also made Jewish Christians especially offended at sharing meals with Gentiles who were accustomed to drink sacrificial blood or use blood in food.