St. Gregory the Great is a doctor of the Church, known for gregorian chant and many liturgical reforms. In 595, he was celebrating the Eucharist, when a miracle happened.
From SSPX.org,
"Pope Gregory was distributing Holy Communion during a Sunday Mass and noticed amongst those in line a woman who had helped make the hosts was laughing. This disturbed him greatly and so he inquired what was the cause of her unusual behavior. The woman replied that the she could not believe how the hosts she had prepared could become the Body and Blood Christ just by the words of consecration.
Hearing this disbelief, St. Gregory refused to give her Communion and prayed that God would enlighten her with the truth. Just after making this plea to God, the pope witnessed some consecrated Hosts (which appeared as bread) change Their appearance into actual flesh and blood. Showing this miracle to the woman, she was moved to repentance for her disbelief and knelt weeping."