The letter you posted is a hoax.
NO it's not.
Here is more material on Darwin - the atheist - and Marx.
According to biologist Ernst Mayr, a founder of neo-Darwinism:
"It is apparent that Darwin lost his faith in the years 1836-39, much of it clearly prior to the reading of Malthus. In order not to hurt the feelings of his friends and of his wife, Darwin often used deistic language in his publications, but much in his Notebooks indicates that by this time he had become a 'materialist.'"