Who decides? Christ.
What part of "This IS My Body" don't you understand?
Unless you want to discuss what the meaning of the word "is" is.
I am not sure where you are coming from. I’m not sure if you are a Catholic or a protestant.
I am a Catholic.
Thus the context of my reply was about what Lutherans believe etc, if you read this to understand the context in which my reply was given.
As I wrote in another comment, there are a great number of independent protestant groups with the same Jesus Christ, God, and Bible, but interpret many very important passages in contradictory ways, in particular about the Eucharist. Thus you say: “Unless you want to discuss what the meaning of the word “is” is.” Yes, the meaning of the word is interpreted in many contradictory ways. As I said before, Jesus knew this would happen and that’s why he left us his One Church with the Holy Spirit to guide the Church and “the gates of hell shall not prevail” (Mt 16:18).
I have a hard time understanding how a Christian does not believe in the real presence of Jesus in the Eucharist after reading chapter 6 of the Gospel of John! After many disciples left because of his teaching on the Eucharist, Jesus did not change his mind as many politicians do to not loose votes. Jesus was even willing to lose his Apostles if they were not willing to accept his teaching without compromise!