Hagiography showed the Eucharist as opaque to non-Christians until explained.

Evidence

But Ampelon recoiled in horror, for what he saw in their hands was not bread but a bloody child, which they were ripping apart and placing into a chalice before sharing it with the assembled faithful. … Ampelon accused him of homicide, telling him what he had seen. The bishop, awed, told the Saracen that he had benefited from a vision; he then explained the mystery of the Eucharist to him. — John V. Tolan, Saracens, 370