Up until his return from the Beagle in 1836, Darwin considered himself an "orthodox" Christian. There is no reason to doubt this although it is important to recognise that his orthodoxy was a specific early 19th century, rational, demonstrable, civilised, gentlemanly kind of orthodoxy. In particular, it was heavily influenced by William Paley whose Natural Theology confidently argued that nature contains "every manifestation of design… [that] design must have had a designer … That designer must have been a person [and] that person is God." Christianity for Darwin was primarily a proof to be established and Paley did that admirably.It had only been recently that new ideas of the age of the earth and coldly factual observations of the geological record had radically revised science's understanding of the earth and its inhabitants. Darwin's faith was not unusual in that respect. As Spencer correctly points out, it was not evolution that separated Darwin from God:
When his emerging theory began to undermine these ideas, it also undermined the Christianity that was built on them. It didn't happen immediately. Darwin's notebooks show him trying to accommodate an intellectually credible idea of God and his new theory – in many ways successfully.
I would posit that a faith that sees evolution as a threat to the very existence of God is not a very strong faith and certainly one that is worth examining.
When, however, his daughter Annie died in 1851, aged 10, suffering moved from being a theoretical problem to an agonisingly personal one. Most Victorian families lost children (Darwin himself lost two others in infancy) but Annie was his favourite and, unlike most Victorian fathers, he had witnessed every last, degrading moment of her short life. The experience destroyed what was left of his Christian faith.
The claim that evolution destroyed Darwin's faith is, thus, only a half-truth, usually made to prove somehow that evolution killed God. By the same reckoning, the claim that evolution had nothing to do with his loss of faith (which was entirely due to Annie's death) is no more accurate, and is often made for equally polemical purposes (usually to demonstrate that evolution presents no challenges whatsoever to religious belief).
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