The Law of Moses is more correctly the Law of God, which God revealed to Moses. Although this law came from God, it’s sometimes attributed to Moses as he was the one who documented it and communicated it to the Israelites.
Among the many “laws” (that is, rules and regulations for right behavior), that God — through Moses — gave the nation of Israel was an unconditional prohibition against drinking blood.
Every Hebrew would have been taught this from early childhood. Breaking this law would have been unthinkable to them, a repulsive act to even consider. Drinking blood was strictly verboten.
Then Jesus came along with his radical teaching that shocked many. He told his followers that they needed to drink his blood. His followers — all Hebrews — were appalled. Viewing his statement as heresy, many turned their backs on him and left.
The idea was so repulsive to them that they were unable to get past the shock of a literal interpretation to consider that it might just have a figurative meaning.
In making this bold statement, Jesus was foreshadowing his sacrificial death. Succinctly, his blood would be spilt as a redeeming, life-restoring sacrifice.
Jesus wasn’t contradicting the laws of Moses. Instead, he voiced his intention to fulfill it.
Two thousand years ago, the Jewish religious scene was comprised of two major factions, the Sadducees and Pharisees. Although their respective theologies about God were quite different, these two camps did coexist – albeit not harmoniously – within the same religion. The reason for their different perspectives of the same God, likely comes from how they treated the Bible.
The short version is that the Sadducees took away from the Bible, considering only the Law of Moses (the first five book of the Bible) as their holy canon. This gave them a partial and incomplete view of God.
The Pharisees did the opposite. They greatly added to the Bible, introducing thousands of their own laws and rules. Although their intension in doing so was to aid them in holy living, their legalistic additions where elevated in importance to the point of surpassing the Bible as their guide.
The error of the Sadducees was to take away from the Bible, while the error of the Pharisees was to add to it. Both are errors that we need to carefully avoid.