Are Catholics required to believe in the Aristotelian "substance and accidents" theory of the Eucharist?

No. Catholics are bound to believe in the Real Presence—that the bread and wine truly become the Body and Blood of Christ—but not in a particular philosophical model explaining how that occurs. Sungenis clarified that “substance and accidents” are categories borrowed from Aristotle’s metaphysics and systematized by St. Thomas Aquinas in the 13th century. Before Aquinas, the Church Fathers used other expressions, like the Greek metousiosis, meaning “change of substance,” to describe the transformation. The Church adopted Aristotelian terminology because it provided an intellectual framework, not because it was revealed truth. Thus, Catholics can affirm the mystery without using that philosophical explanation, as long as they accept the core doctrine of transubstantiation.