How does Abbe reveal to Dantes about a hidden treasure?

The Abbe reveals to Dantes the existence of a hidden treasure on the Island of Monte Cristo. The existence was discovered by the Abbe by reading the will of Caesar Spada, who had been made a Cardinal of Rome. Dantes at first thinks that the Abbe is crazy, but he changes his mind when he reads a letter, which reveals the exact location of the treasure.