The implications of Luther’s discovery were enormous. If salvation comes through faith in Christ alone, the intercession of priests is superfluous. Faith formed and nurtured by the Word of God, written and preached, requires no monks, no masses, and no prayers to the saints. The mediation of the Church of Rome crumbles into insignificance.
As the Roman church had for centuries, Aquinas held that it is a true sacrifice, continuing that of Christ on the cross, and predisposing God to be gracious to those for whom it is offered. In the Supper the essence of the bread and wine are changed miraculously into the actual body and blood of Christ while the exterior remains unchanged, a doctrine known as transubstantiation—of which Thomas gave the classic presentation.