From Why Priests?
The priest is made powerful because of the Eucharist. That explains him. It is what he does, what makes him what he is. No one else can do this – consecrate the Eucharist. But what exactly is the Eucharist? The miracle of changing bread to Jesus is so staggering that many people through the ages have felt they had to explain it, or show its importance, with secondary miracles – like seeing the Host bleed, or an image of Jesus on or above the Host or in the chalice. In legend after legend, people gave the Host visible powers (like levitation). The validity of such Eucharistic miracles was endorsed by theologians during the eleventh-century controversy over the real presence of Christ in the Eucharist. They said that if Christ were not really present in the Host, how could it work such miracles?
Thomas Aquinas saw some danger in these miracles. He felt he had to explain them in such a way that they would not detract from the authenticity of the main miracle, the presence of Jesus in the Eucharist. He noted that some see a spectral Jesus on or over the Host when no one else does, or only in glimpses. Or only an innocent child sees the priest lifting the baby Jesus in his hands, not the Host. He called this a fleeting and subjective visual effect (immutatio oculorum), not affecting the Eucharistic presence. But when a whole group sees a continuing vision of the Lord, he says this is a spectral indication of what is properly present only in the Eucharist – and in that sense to be welcomed, as confirming rather than challenging faith.
Thomas did not try to persuade people that the visions were not there, simply that they did not replace the reality of Jesus, who is present physically only in Heaven, or sacramentally only in the Host and chalice. He must have know there was no way to stop people from supplementing what they believe about the Eucharist with what they could see, or think they see. There are thousands of such ancillary miracle stories around the Eucharist. Sometimes the Host flies from the hand of an unworthy recipient. Sometimes it comes unscathed through fire – like the “miraculous Hosts” of Wilsnack, which made that town in Germany the fourth most popular goal of fifteenth-century pilgrims (after Rome, Jerusalem, and Compostela). Or the Host floats, rather than becomes saturated, in water. Or it paralyzes the tongue of an unworthy recipient. Or a child who has had pagan food in its mouth before receiving the Host has to spit it out. Or it turns to ashes when carried off by someone trying to desecrate it. Or it heals a sick person with its touch. Or, carried in procession, it wards off an enemy attack. Or it was carried out to the fields to cause fertility. It even raised the dead.
Perhaps the most famous story is “the miracle of Bolsena.” In 1263, it is claimed, a priest in Bolsena, Italy, who did not believe in the real presence, broke the large Host held in his hands and the sacred blood ran down onto the corporal (the white linen cloth on which the paten and chalice are placed during Mass). The stained corporal became a prized relic and was installed in a special monstrance at a special altar in the nearby Orvieto cathedral, where it has been worshiped by pilgrims ever since. Still on display, it has been visited by popes. The miracle was commemorated in many paintings, the most famous of which, by Raphael, fills one wall in the Vatican’s Room of Heliodorus. Many red-stained corporals became holy items after that.
The second most famous Eucharistic miracle, and one of the oldest devotions in Italy, is the miracle of Lanciano, dating from about 700 CE. Again a priest who doubted the real presence of Jesus found himself holding a piece of the flesh of Jesus and looking into a chalice with five globules of his blood. The flesh, looked at with a little imagination, is roughly shaped like a heart. It and the five globules of blood are still displayed, in a monstrance for the flesh and a tear-shaped crystal vessel for the drops of blood, to be venerated close up by pilgrims who climb stairs behind the altar in Lanciano. This was so well known an event that Aquinas referred to it in his discourse on secondary miracles, singling out the appearance of “flesh or blood [of Jesus],” caro aut sanguis. He says these cannot be part of the glorified body of Jesus (propria species Christi), which exists only in Heaven. If these were bits of Jesus in his Earthly state, putting them in a ciborium or tabernacle would be putting the glorified Jesus in prison instead of Heaven! Since these apparitions are neither the Heavenly nor the sacramental body, they are not a really present Jesus, says Aquinas, though they are rightly reverenced as aids to piety.