I recently read a Darwin biography by Janet Brown; it comes in two parts,
Voyaging and
the Power of Place. The books are excellent, I highly recommend them.
According to Brown, Darwin never was a firm believer; he was just never really interested in the subject. When he got engaged to Emma Wedgwood, who was a firm believer her entire life, they wrote letters to each other about her concern that he would go to hell. He was very sympathetic to her beliefs and did what he could to comfort her. However, her concern remained throughout their lives together. Also, he was very conscious of the religious sensibilities of Victorian England at the time. He didn't like confrontation, he didn't like offending people. These reasons, among others, are why he waiting so long to publish his theory.
It seems that his beliefs in a Creator at all waned over time. The death of his daughter had a negative impact, and he took no comfort that she was now in heaven as his wife did.
In
the Origin of Species (published in 1859, are you referring to a specific edition??? 1867, would have been like the 6th edition) he avoided as much as he could two subjects; God and the evolution of man. He does mention both briefly in the book, but nothing extensive and really that informative about his thoughts.
I don’t think Darwin really felt that belief and evolution were compatible. He avoided the subject publicly as much he could. He used phrases about the Creator in his books more because that is an idiom people understood, not because that is what he believed.
Two of his closest scientific friends, Asa Gray and Charles Lyell, were believers. Most relevant to your argument probably, he often wrote letters to Gray complaining about Gray explaining evolution as God directed. Darwin maintained that the evidence shows that evolution is random and not directed. (Don’t remember the exact details, darnit, but the point is he did criticize Gray about it.)
His other close scientific friend, Thomas Huxley, was more in line with Darwin's beliefs about God. Huxley, a harsh critic of religion, coined the term agnosticism, and Darwin called himself one. By the time Darwin wrote
Descent of Man and to the end of his days, he was very close to being an atheist.
Edited by Clark, : typo
Edited by Clark, : No reason given.