Who decides if the Eucharist is the true Body and Blood of Christ? The consensus of the majority of human beings? There are thousands of independent protestant groups in the USA, each with the same Bible, the same Jesus, but there are a great number of differences of interpretation about the same passages from the Bible! Where is the authority to give us the Truth that truly sets us free? My opinion? Your opinion? Is it all subjectivism and relativism and thus how can I be sure that I am following the right path that leads to eternal life since I am a subjective human being? Did Jesus Christ, Who is God, not know that there would be this problem, and did He not leave a solution for those with true humility and good will?
Yes, it is called John 17, in Jesus prayer for the disciples.
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.