One day, sitting at the bench outside of Stavronikita, the Elder was visiting with pilgrims, among whom was a high school teacher of theology. The theology teacher, repeating a popular Western error, claimed that Abba Isaac the Syrian was a Nestorian. Father Paisios tried to persuade him that Abba Isaac was not only Orthodox but also a saint, and that his Ascetical Homilies possess great grace and strength. But the Elder’s words were in vain: the theology teacher stubbornly insisted on his views. The Elder left for his hermitage, praying and so sad that he was in tears.
When he had come to a spot on the path near a large plane tree, something happened to him. These words, “something happened”, were the only description he gave us of the incident, not wanting to reveal the exact details. According to one testimony, he saw in a vision the choir of the holy fathers passing before him, and one of them, stopping, said to him, “I am Isaac the Syrian. I am completely Orthodox. The Nestorian heresy was indeed present in my region, but I fought against it.” We are not in a position to endorse or to reject the reliability of this witness. We know for certain only that the Elder experienced a supranatural occurence that confirmed with perfect clarity the holiness and total Orthodoxy of Abba Isaac.
(Elder Paisios of Mount Athos by Hieromonk Isaac pg. 226)