Once, when I was about to communicate, I saw, with the eyes of the soul, more clearly than ever I could with those of the body, two devils of most hideous aspect. Their horns seemed to be around the poor priest's throat; and when I saw my Lord, with the Majesty which I have described, in the hands of such a man, in the Host which he was about to give me, I knew for a certainty that those hands had offended Him and realized that there was a soul in mortal sin... This upset me so much that I do not know how I was able to communicate, and I was sore afraid, for, I thought, had it been a vision from God, His Majesty would not have allowed me to see the evil that was in that soul. Then the Lord Himself told me to pray for him and said He had allowed me to see this so that I might realize what power there was in the words of consecration, and that, however wicked the priest who pronounces those words may be, God is always present without fail. He wanted me also to appreciate His great goodness in placing Himself in the hands of that enemy of His, and this solely for my good and for the good of all. This showed me clearly how much stricter is the obligation laid upon priests to be virtuous than upon other people, and what a terrible thing it is to take this Most Holy Sacrament unworthily, and how complete is the devil's dominion over the soul that is in mortal sin. It was of the very greatest help to me and gave me the fullest knowledge of what I owe to God. May He be blessed for ever and ever.

The Autobiography of St. Teresa of Avila
St. Teresa of Avila