Rabbi Nissan Zaks recounted:

A rather peculiar man lived in London during the time that Rav Kook served in the local rabbinate. What was so peculiar about him? He thought he was Mashiach (the Messiah). His relatives brought him to psychologists and even turned to Jewish 'miracle workers,' but to no avail. No one was able to rid him of his insanity.

Finally, they brought him to Rav Kook. Amazingly, after a brief meeting in private, the rabbi was able to cure him of his malady.

How did Rav Kook achieve such remakable results? The Rav later revealed what he had told the man:

"You should know that you are right. To a certain degree, you are Mashiach, for every Jew has a Messianic spark in him. Some have a small spark, and some — like you — have a large one.

"However, this spark is only valuable if it is kept secret. If you speak about it, you might lose it, God forbid."

(from 'An Angel Among Men' by R. Simcha Raz, translated by R. Moshe Lichtman, p. 354)