I didn't say anything about Christianity. I said:
Unless you mean a handful of letter to Fiske being the sum total of "last works," which is rather an eccentric view.
You've clarified you also include The Variation of Animals and Plants Under Domestication as a "last work," but this is odder still. Variation was published in 1868, before The Descent of Man (1871), The Expression of the Emotions in Man and Animals (1872), and Insectivorous Plants (1875).
Darwin did come out with a second edition of Variation in 1875, but this in turn was followed by entirely new books such as The Various Contrivances by which Orchids are Fertilized by Insects (1876), The Effects of Cross and Self Fertilization in the Vegetable Kingdom (1876), and several other book length works. Darwin's last book was The Formation of Vegetable Mould, through the Actions of Worms, with Observations on Their Habits, published in 1881, just a year before he died.
You've engaged in a great deal of arm-waving and cryptic references, but you've never pointed to any words by Darwin himself in support of your contention that he remained a Christian, or returned to Christianity, or himself interpreted his views on evolution in any sort of theistic context. Why don't you quit the zen sage routine and present some actual evidence. If you think Variation is so important in this respect, you can find full text of the 2nd (last) edition here. Cut and paste the passages you consider relevant.