Many Protestants and those from other Christian faith traditions constantly ask Catholics why they are unable to receive the Eucharist at a Catholic mass - that is, partake in the Lord's Supper, where we receive the true Body and Blood of Jesus Christ, as outlined in John 6:53 from the Bible. However, most Catholics do not believe in the real presence of the Eucharist through transubstantiation, and thus should not receive either.
