Here is his letter to John Fordyce, dated May 7, 1879:
Down Beckenham | Kent
May 7th 1879
It seems to me absurd to doubt that a man may be an ardent Theist & an evolutionist.— You are right about Kingsley. Asa Gray, the eminent botanist, is another case in point— What my own views may be is a question of no consequence to any one except myself.— But as you ask, I may state that my judgment often fluctuates. Moreover whether a man deserves to be called a theist depends on the definition of the term: which is much too large a subject for a note. In my most extreme fluctuations I have never been an atheist in the sense of denying the existence of a God.— I think that generally (& more and more so as I grow older) but not always, that an agnostic would be the most correct description of my state of mind.
Dear Sir | Yours faithfully | Ch. Darwin
Here is the link to the Darwin Correspondence Project.