This is a slim but informative book on Darwin's religion- Darwin began as a Unitarian whose ideas, like those of many educated persons in his time, were based on William Paley's 'Natural Theology' of benevolent design. Consequently, the discovery that design did not appear all that benevolent had a thinning effect on a blood of faith that did not flow particularly strongly in his veins anyway. The cruel death of a favourite child seems to have had a final, chilling effect. But the evidence is that he was never much interested in spiritual questions, and was always something of a Laodicean.
The argument can be summarised as follows:
(a) Darwin came from a family of whig unitarians;
(b) such 'faith' as he started with depended on the scientific rationalism of William Paley, and, in particular, that the existence and nature of God could be inferred from what appeared to be the orderly and benevolent dsign and construction of the material world;
(c) that despite considering ordination, Darwin couldn't be bothered to read the gospels and had to ask his sister, when she proposed the he should do so, what the 'best' one was;
(d) that his loss of 'faith' did not amount to much more than the substitution of one set of 'rational' tenets for another;
(e) that the surrender of his specifically Christian beliefs was more or less directly occasioned by the cruel suffering and death of a favourite daughter;
(f) that, as a result of his rationalism, he never made a distinction between gentlemanly, or decent, and Christian, behaviour;
(g) that he remained puzzled at the discrepancy between the apparently soulless mechanism of natural laws, and their convergence, which suggested to him that the universe was not the result of chance;
(h) that he regarded the mind of man as necessarily adapted to the immediate circumstances of its environment and therefore unreliable in the context of speculation extraneous to those circumstances: hence and in contrast to some of the would-be wearers of his mantle.
(g), a decent kind of abstention from the debates which his theory occasioned, and a kindly benevolence towards the less sceptically inclined of his contemporaries.
His wife, Emma once wrote to him: 'May not the habit in scientific pursuits of believing nothing till it is proved, influence your mind too much in other things which cannot be proved in the same way, & which, if true, are likely to be above our comprehension?'
Which seems to me to put it with admirable succinctness: one should not expect to observe the sun's light when one is working a mile underground.