Elder Joseph the Hesychast. "Monastic wisdom". From letter 16.
As for the brother you wrote about who is suffering, we shall say prayers on his behalf. He must be committing some hidden sin now or in the past, and for this reason God allows him to suffer in this way. Many times a person commits a sin without realizing that he is sinning by doing it. He walks like a blind man in the darkness, without the courage to see himself in the light and to say, “I have sinned, my God!”