From the supper at Emmaus, disciples have cherished the Eucharist as the clearest sign of the Risen Lord's abiding presence.

Yet, beginning in the Middle Ages, for a thousand years laypeople in the West seldom received Communion, and even then only under the form of bread. For that reason knowing when Christ became present at Mass was a key to spiritual life, since the ordinary way of worship was looking at the consecrated Host raised high over the priest's head rather than eating it. Medieval scholars-all priests-naturally looked at the Eucharistic Prayer and the cherished memory of the words and deeds of Jesus at the Last Supper, called the "Institution Narrative." The winning answer in the debate was keyed to the words of the priest: "This is my body." (Hoc est enim...