This delightful gem from the Desert Fathers comes from Benedicta Ward’s The Wisdom of the Desert Fathers (157)
There was a saint in Egypt who dwelt in a desert place. Far away from him there was a Manichean who was a priest (at least what they call a priest). Once, when this man was going to visit one of his confederates, night overtook him in the place where the orthodox saint was living. He was in great distress, fearing to go to him to sleep there, for he knew that he was known as a Manichean, and he was afraid he would not be received. However, finding himself compelled to do so, he knocked; and the old man opened the door to him, recognized him, received him joyfully, constrained him to pray, and after having given him refreshment, he made him sleep. Thinking this over during the night, the Manichean said, “How is it that he is without any suspicions about me? Truly, this man is of God.” And he threw himself at his feet, saying, “Henceforth, I am orthodox,” and he stayed with him.