IOW, it is right down there with being baptized in terms of completely irrelevant to the question of whether Darwin was a Christian.
At least in the tradition within which I was raised, which in this respect was similar to Darwin's own, baptism is not "irrelevant" to the question of one's religious faith. I see where from a post-Russell style atheist perspective, the only thing that could possibly matter is whether one "believes" the "supernatural truth claims" of non-atheist traditions. But unless we actually establish that Darwin thought of himself as an atheist, and in the modern sense besides, what bearing does this internal bias toward ontology have on the discussion?
In Anglican thought, the tradition to which Darwin at some points in his life at least belonged (and when did we agree that deathbed beliefs are the only important kind, anyway?) sacraments are major business, not easily undone. Having doubts about the veracity of this or that specific doctrine does not automatically undo baptism any more than fucking a stranger automatically cancels a marriage.
Providing a letter which suggests doubt about something certainly does not prove that another person is outside of God's grace; this is like saying your marriage is canceled if your mother heard that someone saw you flirting with someone.
I do not think Darwin, in fact, invited any sort of public pondering of his religious outlook. Knowing, as he must have, that any such public pronouncements would unfortunately affect the acceptance or rejection of his central hypothesis for non-scientific reasons, he kept such thoughts close to his chest, sharing them only with close confidants and that only in morsels and hints that could be interpreted flexibly. There is every reason in the world to believe that his positions on things changed over time. If he was at any point an atheist in the sense that you all mean it, then he was at the most a Christian
and an atheist at certain different points in time. And I am not convinced that he was ever a Richard Dawkins type secretly on the inside, etc; his writings are very consistent with the sort of skeptical deism common in intellectual circles in his day, and I think this more likely. Combine this with a largely Christian social life, and I think it is
reasonable to call Darwin by any number of labels, and
wiser to avoid labeling him at all, as indeed was (very sensibly IMO) his wish to begin with.
I also really don't care about anyone's conclusions outside the folly of the OP in trying to go all radical-or-bust about them. As a fact by itself, I don't decide what to believe or not to believe based on what Charles Darwin did or did not believe, much though I greatly respect the man. Not just as a biologist either. But he isn't my guru.
