A LETTER that kept the secret of Charles Darwin’s faith for almost a century after his death is predicted to sell for up to £60,000 when it goes to auction this month.

The question whether he believed in the Bible, and in Jesus Christ as the son of God, was a source of speculation in his lifetime. But the scientist, who read theology at Christ’s College, Cambridge, always avoided discussing his beliefs — or lack of them.

He told the prominent atheist Edward Aveling: "It has . . . been always my object to avoid writing on religion, and I have confined myself to science."

But in November 1880, a young barrister, Francis McDermott, wrote to him, asking: "If I am to have pleasure in reading your books, I must feel that at the end I shall not have lost my faith in the New Testament. My reason in writing to you therefore is to ask you to give me a Yes or No to the question ‘Do you believe in the New Testament?’" He promised not to publicise Darwin’s reply in the "theological papers".

Darwin replied: "Dear Sir, I am sorry to have to inform you that I do not believe in the Bible as a divine revelation & therefore not in Jesus Christ as the son of God. Yours faithfully, Ch. Darwin."

Rumours that he returned to faith two years later, on his deathbed, were denied by his daughter.

McDermott kept his promise, and Darwin’s response remained unknown to scholars until late into the 20th century. It comes up for sale in the History of Science and Technology auction at Bonhams, in New York, on 21 September.