After all, Darwin had studied theology and was interested in all kinds of human behaviors. Thus, it doesn't come as a surprise that he dedicated sections of his works to the evolution of religious beliefs and behaviors. In his eminent "Descent of Man" (1871) he wrote (Ch. 2, p. 65):
“Belief in God—Religion.—There is no evidence that man was aboriginally endowed with the ennobling belief in the existence of an Omnipotent God. On the contrary there is ample evidence, derived not from hasty travellers, but from men who have long resided with savages, that numerous races have existed and still exist, who have no idea of one or more gods, and who have no words in their languages to express such an idea. The question is of course wholly distinct from that higher one, whether there exists a Creator and Ruler of the universe; and this has been answered in the affirmative by the highest intellects that have ever lived.
If, however, we include under the term "religion" the belief in unseen or spiritual agencies, the case is wholly different; for this belief seems to be almost universal with the less civilised races. Nor is it difficult to comprehend how it arose.“
Darwin is here (I'd say rightfully!) advocating methodological agnosticism - the evolutionary exploration of religion(s) is neither proof nor denial of God's existence. What's more, he is distinguishing the universal biological trait of religiosity (as belief in supernatural agents) from its cultural forms, e.g. as poly- or monotheism. In fact, he is ascribing the latter explicitly to cultural evolution (Ch. 21, p.395):
“The idea of a universal and beneficent Creator of the universe does not seem to arise in the mind of man, until he has been elevated by long-continued culture.“
Following a lead by David Hume (which I plan to explore in another blogpost), Darwin was convinced that it was not difficult to comprehend how religiosity arose! He assumed the emergence of animistic beliefs borne out of mental faculties which we name today as HAD (Hyper Agency Detection) and TOM (Theory of Mind) (Ch. 2, p. 66):
“I cannot but suspect that there is a still earlier and ruder stage, when anything which manifests power or movement is thought to be endowed with some form of life, and with mental faculties analogous to our own.“
And Darwin took his dog (and an Ape in another chapter) as comparative examples for pre-religious traits, e.g. in Ch. 2, p. 67:
“My dog, a full-grown and very sensible animal, was lying on the lawn during a hot and still day; but at a little distance a slight breeze occasionally moved an open parasol, which would have been wholly disregarded by the dog, had any one stood near it. As it was, every time that the parasol slightly moved, the dog growled fiercely and barked. He must, I think, have reasoned to himself in a rapid and unconscious manner, that movement without any apparent cause indicated the presence of some strange living agent, and no stranger had a right to be on his territory.”
From here, the evolutionary path seemed pretty clear to him, as he -again- emphasized what we call TOM today (Ch. 2, p. 67):
“The belief in spiritual agencies would easily pass into the belief in the existence of one or more gods. For savages would naturally attribute to spirits the same passions, the same love of vengeance or simplest form of justice, and the same affections which they themselves experienced.”
And although Darwin became an agnostic in his later years (while other eminent evolutionist as Alfred Russel Wallace or Antoinette Brown Blackwell retained their religious beliefs), he included religion in his hopefull view of evolutionary progress (!), Ch. 5, p. 184:
“It is apparently a truer and more cheerful view that progress has been much more general than retrogression; that man has risen, though by slow and interrupted steps, from a lowly condition to the highest standard as yet attained by him in knowledge, morals, and religion.“
To me, it seems to be pretty clear that Darwin would applaud our recent findings e.g. of a partial, genetic heritability of religiosity, the signalling function of religious behaviors, the evolutionary psychology of religious symbolism or the reproductive advantages by religious believers as confirmations of his evolutionary theory. And maybe he would be disturbed by those authors as e.g. Richard Dawkins claiming to have written "Darwinian" books about religiosity and religions - without including any discussions of those original hypotheses Darwin devised himself.