The following is based on an article in the London Jewish Chronicle (PDF) by Avraham Melnikoff (1892-1960), a sculptor from Jerusalem who lived in London from 1933-1959.
The article was written two weeks after Rav Kook’s passing, and the author recounts a conversation that he had with Rav Kook, where the latter told Melnikoff: “When I lived in London I used to visit the National Gallery, and my favourite pictures were those of Rembrandt. I really think that Rembrandt was a Tzaddik. Do you know that when I first saw Rembrandt’s works, they reminded me of the legend about the creation of light? We are told that when God created light, it was so strong and pellucid, that one could see from one and of the world to the other, but God was afraid that the wicked might abuse it. What did He do? He reserved that light for the righteous when the Messiah should come. But now and then there are great men who are blessed and privileged to see it. I think that Rembrandt was one of them, and the light in his pictures is the very light that was originally created by God Almighty.”
Melnikoff concludes that he had “read much about Rembrandt, but none gives such a vivid description of his genius... Only a man as pure of heart and soul as Rabbi Kook could have seen Rembrandt in that light.” (London Jewish Chronicle, September 13, 1935, p. 21.)