Would Christianity have been in better shape today if Darwin had never been born? It suits both many atheists and millions of believers to suppose that the answer is obviously "yes" – but if the answer is obvious at all, then theirs is the wrong one. Obviously Christianity today much less credible among intellectuals than it was in 1600, and this is largely a result of the growth of our knowledge about the world since then. But when you look at the thinkers whose contributions did most to make literalist faith in the Bible incredible, Darwin was really not one of the most important: in fact, as the welcome extended to his ideas by many Victorian Christians showed, his theory offers a way around the difficulties raised by others.
In any atheist pantheon, the credit for pulling down the house of faith needs to be shared quite widely, but at least half of it belongs to devout Christians. It is simply not the case that "science" showed "religion" was incredible. Neither science nor religion are single, simple entities like that; in any case the slow decay of Christianity's credibility was a result of developments in philosophy, in history, in physics and in geology long before Darwin.
By the time that Darwin published the On the Origin of Species in 1859, it was already obvious that the God of the Bible was being squeezed right out of the educated world view. The physical world was increasingly revealed as law-bound; and Hume had argued that miracles (pdf)had to be understood as breaches of these natural laws, to be credited only when no other explanation was possible. The belief in the workings of providence in history could not among intellectuals easily survive the study of Gibbon and Voltaire. The literal truth of the Biblical narratives and even the credibility of their perspective on history had already been destroyed by the geologists' discovery of the unimaginable age of the earth.
All this was true – and fatal to traditional Christianity – before Darwin published a line. The only theist argument that his work destroyed was the argument from design. But the argument from design is of interest only to nerds, whether atheist or believers. Most people just don't have the kind of systematising imaginations which make the question of design in nature look compelling; other forms of imagination, while they marvel at the complexities of living things, don't see why this should not be the work of a God responsible for the laws of natural selection.
If you concede – as the majority of Christians do without trouble – that God may express his purposes through the laws of the evolution, then Darwin's discoveries do nothing to diminish the fervour of the believer: as his son Francis wrote:
Asa Gray observed that if the orchid book "had appeared before the Origin, the author would have been canonised rather than anathematised by the natural theologians."
What made Darwin threatening to Christianity was not that he abolished the argument from design, but that he threatened – and threatens – human uniqueness. Against this, though, two points can be raised. The first is that Darwinian explanations of humanity end up with accounts of us which are much more compatible with the Christian view of human beings as inherently sinful and "fallen" than is the simple faith in human moral progress that was a powerful alternative to Christianity. The second is that Darwin lets God off the hook for much of the suffering of the natural world.
The more we understand about the workings of biology, the more horrible much of life appears. Most of it is parasitic; most of it is unremittingly ruthless; all of it is doomed. Tennyson called nature "red in tooth and claw" in 1843, 16 years before Darwin published the Origin of Species. If God had personally designed every last parasitic wasp and tapeworm: if some celestial watchmaker had carefully sculpted the HIV virus to make it so effective, and had shaped Eve to make her die so often in childbirth, then the case against him would be morally quite unanswerable, as Voltaire saw.
Darwin's theory allows Christians – whether they want to or not – to understand the hideous and constant cruelties of the world as part of the mechanisms necessary to produce any kind of intelligent life. Disease, decay and death need no longer be exhibitions of gratuitous cruelty on the part of a creator. This isn't by any means a knock-down argument for belief. But it is a conclusive argument against one kind of morally outrageous god.
Without Darwin this defence would not have been possible – but there is one final twist in the argument. It was not his personality, but his discovery which changed the rules of the world, and since natural selection is a fact about the world, and not a theory, it would have been discovered if he had never been born; in fact it was discovered, quite independently, by Alfred Russell Wallace – and Wallace was a spiritualist in his old age.