Thomas of Kempis tells the following story:
A youth who attended Mass was unable to to see the Host—try as he might. Because he ascribed this to weak eyes and to distance from the altar, he got as near as possible. Still he could not see the Sacred Host. This went on for some time, until, at length, disquieted he told his confessor about it. The confessor questioned him carefully and finally found out that the young man bore hatred towards a companion and would not forgive him. That cleared up matters.
“I see,” he said, “you bear a grudge against someone, and this is the reason why the Sacred Host is taken from your sight. For, because you are so wanting in love for your neighbor, the Lord in His mercy wishes to show you that though you attend the holy Sacrifice you derive no benefit from it as long as you are in that condition. Put away your hatred and God will take pity on you.”
He promised—received absolution—and lo! the Sacred Host became visible again in the priest’s hands. — The Liguorian, Vol. XIL, No. 9, September, 1924