Let me put this plainly: there is no direct documentary evidence for this claim from anything Darwin himself ever wrote. This is an inference (as is TB as a cause of Annie's death) that has somehow hardened into a "fact" in the last decade or so, starting with the "rediscovery" of Annie's grave in Malvern by Jim Moore and amplified by Randal Keynes' book Annie's Box.
Now, in case you missed my point there: there is NO evidence for the idea that Darwin's abandonment of Christianity was influenced by the death of his daughter or that this event precipitated an agonising crisis of faith. If anyone of you out there have some, then let's have it. But I cannot find a single jot. If this really were the case, wouldn't Darwin have said so somewhere? His life is among the best-documented of all scientists, but there is nothing there to support this claim! Take a look at what he says on this topic in his Autobiography:
"Thus disbelief crept over me at a very slow rate, but was at last complete. The rate was so slow that I felt no distress, and have never since doubted even for a single second that my conclusion was correct. I can indeed hardly see how anyone ought to wish Christianity to be true; for if so the plain language of the text seems to show that the men who do not believe, and this would include my Father, Brother and almost all my best friends, will be everlastingly punished. And this is a damnable doctrine."
No mention of Annie's death there!
So, please let's pull back from continually asserting as fact something that is mere supposition. It is entirely possible that Annie's death had an impact on Darwin's religious beliefs, but there is no documentary evidence for this! I remain an Annie-sceptic! If you disagree, show us your evidence!