According to CBS News:
While many scholars believe Charles Darwin was an agnostic or even an atheist, it can be difficult to find hard evidence to back up those beliefs. He was reluctant to discuss religion and his writings are often silent on the issue.
But now a simple one-sentence handwritten letter signed by the naturalist offers proof he did not believe in God. The letter, written 21 years after the publication of “The Origin of Species,” was auctioned off Monday afternoon at Bonhams in New York. It fetched $197,000 at the auction, three times the previous record of $59,142 for a four-page letter that Darwin had penned to his niece.