St. Thomas said the Eucharist is the one instance of change we encounter in this world that is exactly the opposite. The appearances of bread and wine stay the same, but the very essence or substance of these realities, which can’t be detected by a microscope, is totally transformed. What was once bread and wine are now Christ’s body and blood. A handy word was coined to describe this unique change. Transformation of the “sub-stance,” what “stands under” the surface, came to be called “transubstantiation.”
What makes this happen? The power of God’s Spirit and Word. After praying for the Spirit to come (epiclesis), the priest, who stands in the place of Christ, repeats the words of the God-man: “This is my Body; This is my Blood.” Sounds to me like Genesis 1: the mighty wind (read “Spirit”) whips over the surface of the water and God’s Word resounds. “Let there be light” and there was light. It is no harder to believe in transubstantiation than to believe in Creation.