Archimandrite Raphael. "From time to eternity. Sayings of the Fathers".
Hieromonk Vasily (Pirtskhalava) said: “The worst sin for a monk is fornication. Other sins, compared to it, are dust that can be easily blown away.”
He said: “I heard from an elder who lived for many years on Mount Athos that a priest who commits fornication and at the same time serves Liturgies is in hell even before death. When he proclaims: "Peace to all" and "The blessing of the Lord is upon you," the Angel of the altar says: "Blessing from God to the people, and to you - a curse." When he takes communion, his soul turns black, like burnt coal. When he finishes serving the Liturgy, the devil comes up to him and kisses him on the lips as his friend.”
He said: “Two monk brothers lived not far from each other. One went to the city on some business and fell there. And the other saw in a dream: a seething sea, his brother-monk was walking on a slippery rock and suddenly fell into the abyss. He began to scream in his sleep: “Help!”, but the sea drowned out his voice.”
I asked Hieromonk Vasily: “Is there a sin worse than this sin?”—“Despair and suicide,” he answered.—But most often it is fornication that leads a monk to despair.” “And why is a monk allowed to fall into fornication, because he prays to God and wants to be saved?” — “For disobedience and unconfessed sins.” I asked again: “Can this sin be forgiven for the priest?” “Let the bishop decide this,” he replied.