Siman 173

(7.) And I myself already heard from his holy mouth that he said regarding the aforementioned book — that a tzaddik alone will not understand anything in it, and similarly a sage alone will not understand it either — only a tzaddik who is also a sage: namely, one who is a very great tzaddik and also wise in all seven wisdoms. But even so, in any case someone will be found who understands it. But the Hidden Book — which is of the aspect of razin d'razin compared to him — this book has never been seen by any eye in the world. And he said that in this book no person in the world will understand anything at all — only I heard in his name that he said Messiah will explain it as mentioned above.