V.v.38. There was a certain brother troubled with sexual thoughts who happened to see the daughter of a pagan priest as he was going into an Egyptian village. He fell in love with her and asked her father to give her to him for a wife. “I can’t do that,” he replied, “without asking my god.” So he went to the demon whose cult he served and said, “Look, this monk has come to me, wanting to have my daughter. Shall I give her to him?”
The demon answered, “Ask him if he will renounce God and his Baptism and his monastic way of life.” Going back to the monk he said, “Deny your God and your Baptism, and your monastic way of life, and I will give you my daughter,” to which he agreed.
And immediately the priest saw a dove come out of his mouth and fly up to heaven. He went back to the demon and said, “See now, he’s promised those three things”
But the devil replied, “You still can’t give him your daughter because God has not abandoned him. He helps him still.”
The priest went back to the monk and said, “I still can’t give her to you for your God still helps you and has not departed from you.”
When the brother heard this he said to himself, “If God has shown me such grace, when I have ungratefully denied him and my Baptism and my monastic way of life, if he in his goodness has even now come to my help in my wickedness, why am I departing from him?”
And he turned himself around and came to his senses, and went back to the desert to a certain respected old man to whom he confessed all. And the old man said, “Sit with me in the cave and fast with me for three weeks and I will pray to God for you.”
And the old man agonised for his brother and prayed God, saying, “O Lord, I pray you, grant me this soul, and accept his penance.”
And God heard his prayer. At the end of the first week the old man went to the brother and asked him whether he had seen anything, and the brother replied that he had seen a dove hovering high up in the heavens above his head. And the old man said, “Don’t relax, keep on praying earnestly to God.”
At the end of the second week the old man came to him again and asked him if he had seen anything, and the brother replied that he had seen the dove coming down towards his head.
The old man replied, “Keep your mind alert and pray.”
At the end of the third week the old man came to him again and asked whether he had seen anything more. He replied, “I saw the dove come and stand right over my head, and as I reached out my hand to take it, it flew right into my mouth.
And the old man gave thanks to God, saying, “See, the Lord has accepted your penitence. Now you must watch over yourself and be vigilant.”
The brother replied, “I shall stay with you till I die.”
From De Vitis Patrum – Book V – Libellus 5: Sexual Temptation