Understanding the Eucharistic Miracle of Transubstantiation Transubstantiation is the Catholic doctrine that during the consecration of the Eucharist, the bread and wine used in the sacrament become the actual body and blood of Jesus Christ, while retaining only the appearances of bread and wine. This doctrine is based on the teachings of St. Thomas Aquinas, a prominent theologian and philosopher of the Catholic Church. St. Thomas Aquinas explained transubstantiation using the philosophical fram