There was once a pious scholar who left behind a son, Rabbi Isaac, greater in learning and piety than himself, and a dayyan — a judge in the Jewish court.
On the eve of Rosh Hashanah, Isaac's dead father appeared to him in a dream and gave a command so strange that it threw the son into turmoil: convert, tomorrow, to the king's religion. Isaac woke weeping. He put on sackcloth. He fasted three days.
On the eve of Yom Kippur the father appeared again, this time angrily. "Why have you not obeyed me?" Isaac argued. He begged. He refused. The father repeated: tomorrow — on Yom Kippur itself — you must present yourself to the king and convert.
Isaac did not sleep that night. In the synagogue the next day he wept so bitterly that the whole congregation noticed. They pressed him for a reason. Finally, after long persuasion, he told them. The congregation wept, because they knew that once Isaac was gone, they had no defender, no advocate before the king's court, no one to speak for them when accusations came. They fasted with him.
Isaac went before the king. "I will accept your religion," he said, "on one condition: that I may return to my own faith whenever I choose." The king, amused, agreed.
The king was old and had one heir — a beloved son. There was in the court a powerful prince whose three sons resented the succession. They murdered the heir. The Jews were accused of the killing. It was the oldest accusation in the book, and it would have ended in a pogrom.
Isaac asked to be taken to the cemetery where the prince was buried. At the grave, he used his knowledge — the tradition is vague here, but suggests a prayer or a mystical act — and he caused the murdered prince himself to speak from the dead. The corpse named the three real killers.
The king executed the murderers. The accusation against the Jews collapsed. And Isaac, his work done, exercised the clause he had negotiated and returned to Judaism as a great rabbi, honored by the king.
Gaster's Exempla (no. 339, 1924; from Codex Gaster 66) preserves this medieval tale because it captures something Jews in exile knew in their bones: sometimes the only way to save a community is to walk briefly, and painfully, into enemy territory.