Some say he was not. Some yes..
He wrote that he was an "Agnostic" in the 1870s (Nora Barlow [ed.], The Autobiography of Charles Darwin).
Despite Darwin's own writings, James Moore, an authority on Darwin's life, suggests that if one's religion is truly reflected by the way one lives out one's life, and not merely by one's own writings, then there is a good case that Darwin was a believer, although his belief did not conform to the prevalent conception of Christianity back then, given Darwin's exposure and surrounding. (Read Moore's short essay 'That Evolution Destroyed Darwin's Faith in Christianity-Until He Reconverted on His Deathbed', in Galileo Goes to Jail, p.142).
Throughout Darwin's life, he was actively involved in the works of his local church. Even in the late 1878, John B. Innes wrote to Darwin (they are life-long friends), explaining to him how he expressed his view on the relation between natural sciences and religion. And in the letter, we know that Darwin was not hostile to Christianity (at least in his writings and in Innes' presence).
Read the recent article by John Hendley Brooke, which was written on request by the International Society for Science & Religion.
From these, we can be certain that Darwin:
- Was an agnostic.
- Was not hostile to the belief in Christianity.
- Though agnostic, he was involved in the social works of the local church and other Christianity-related initiated works. Hence we can affirm that he at least acknowledged the good of these initations.
This attitude is very different from the New Atheists, who always domesticate Darwin's works as weapon against religion.