1: The doctrine that neither the nature nor the existence of God, nor the ultimate character of the Universe (that is, whether it is material or ideal), is knowable. This doctrine was formulated by Huxley to distinguish his position from atheism, which positively rejects belief in God's existence.
Please note, it doesn't deny that God exists, only that humans can not know him whether he does or not. But that's only one definition, and Huxley's, which Darwin did not buy into.
2: Any doctrine, which while professing belief in God's existence, denies to a greater or lesser extent the knowableness of his nature. Thus, Mansel held that man is compelled to believe in God's infinite being though he is unable to comprehend it. Spencer's agnosticism is of this type, affirming, as it does, the existence of an Unknowable.
Darwin thought better of Spencer overall, but he didn't buy this one either, and he was very clear on the point in his letters to Fiske, and Fiske, after his discussions with Darwin, so too. .
3: Any doctrine which affirms the impossibility of any true knowledge from the fact that all knowledge is relative and uncertain. It may arise from belief in the relativity of knowledge either as revealed in preception of sensible phenomena, or as shown in the element of error in abstract conceptions.
This is nice too, but it doesn't approach any of the key issues in Fiske's work on Darwin and theology on the matter. Simply put, Darwin had some serious doubts about how God worked in the world, particularly in light of sin, and above all, the major issue of the day, slavery. Note that in England, in the previous century the list of those who indulged in the business of fleshmongering included everyone who was anyone, and this bothered Darwin's rather traditional notion of sin and repentence regarding his various ancestors and their relatives.
Fiske, of course, deals with this a good bit in his historical work, and in his interpretations of the religous implications of history in light of Darwin's theory.
Now, here is the nut of your problem. Those guys you are reading were largely typical of their era and largely racist. As for Darwin's autobiography, it was edited by one of his female relatives and published sometime after his death. To what extent he was actually involved with it in his final years is questionable, but we do know that the editor was a Marxist and not particularly Christian, and Darwin did note that before his death. It's not surprising you mention the old saw of his alleged death bed conversion. This story only popped up after the Marxists started printing that he didn't believe in God. Do you boys always believe everything you find printed by Marxists? Really? You would be surprised how many University Marxists edit third party books, or abridge first party books. The particular Marxist story of Darwin's denial of Christianity comes from a Marxist who once came to his house and was soon shown the door, and the bedside conversion comes from a similar witness. The comments of Fiske come from spending months living in Darwins house with Darwin. I count that as a difference in quality, you may dispute it, but I won't buy that disputation.
But then, we are back to where we started, when I noted that Darwin considered that intimate knowledge of natural selection in the world of wild animals (not domesticated ones - including man) to intimate knowledge of God in the world. Fiske also, and both of them came to the conclusion that not only does God live in the world, but that there is no such thing as free will. By that they meant that everything that happens, happens because God wills it, it is his plan, and his mystery. Including man and his works. Chew on that a bit.
Is your Christian faith as strong as Darwin's alleged Agnosticism? (yes, Darwin used the word on occasion, but like an agnostic, we can't ever be sure exactly just what he intended in those couple of instances where he used it) Note, Fiske was definitely not an Agnostic, and Fiske had no issues with Darwin around that alleged discrepancy.
What in the world are you on about? Darwin's denial of Christianity is in his autobiography, in his own hand.