The answer
For the first thousand years of Christian history, the holy gifts of the Body and Blood of our Lord Jesus Christ were received as just that: His Body and Blood. The Church confesses that Eucharist is a mystery: The bread is truly His Body, and that which is in the cup is truly His Blood. However, one does not know to be able to explain how they have become so. It is a mystery.The eleventh and twelfth centuries brought about the scholastic era, the Age of Reason in the West. The Roman Church, which had become separated from the Orthodox Church, was pressed by rationalists to define how the transformation takes place. They answered with the word transubstantiation, meaning a change of substance. The elements are no longer bread and wine; they are physically changed into flesh and blood. Thus the sacrament, which is to be accepted by faith, was subjected to a philosophical definition.
This second view of the Eucharist, the consubstantiation of Martin Luther, was unknown to the ancient Church. Our Orthodox Church teaches that the material elements of the Eucharist (bread and wine) become grace-filled by the calling of the Holy Spirit. We call the Eucharist "the mystical Supper." What the priest and the believers partake of is mystically the real Body and Blood of our Lord Jesus Christ. We receive Him under the forms of bread and wine.
Answered by His Eminence Metropolitan Youssef (opens in a new tab)