A couple of years ago, I read a collection of Darwin's personal letters and autobiography, compiled and edited by his son Francis Darwin. I'm considering buying a copy because it's really such a wonderful read. In the meantime, I've found several online sites dedicated to hosting the texts.
I've excerpted a few of the tasty bits Darwin wrote on religion, but please, if you're in the market for a new book, I highly recommend either picking up a copy or reading the autobiography online.
Whilst on board the Beagle I was quite orthodox, and I remember being heartily laughed at by several of the officers (though themselves orthodox) for quoting the Bible as an unanswerable authority on some point of morality. I suppose it was the noveltry of the argument that amused them. But I had gradually come, by this time, to see that the Old Testament from its manifestly false history of the world, with the Tower of Babel, the rainbow at sign, etc., etc., and from its attributing to God the feelings of a revengeful tyrant, was no more to be trusted than the sacred books of the Hindoos, or the beliefs of any barbarian.
By further reflecting that the clearest evidence would be requisite to make any sane man believe in the miracles by which Christianity is suppoted, -- that the more we know of the fixed laws of nature the more incredible do miracles become, -- that the men at that time were ignorant and credulous to a degree almost incomprehensible by us, -- that the Gospels cannot be proved to have been written simultaneous with the events, -- that they differ in many important details, far too important as it seemed to me to be admitted as the usual inaccuracies of eyewitnesses; -- by such reflections as these, which I give not as having the least noveltry or value, but as they influenced me, I gradually came to disbelieve in Christianity as a divine revelation. The fact that many false religions have spread over large portions of the earth like wild-fire had some weight on me. Beautiful as is the morality of the New Testament, it can hardly be denied that its perfection depends in part on the interpretation which we now put on metaphors and allegories.
That there is much suffering in he world no one disputes. Some have attempted to explain this in reference to man by imagining that it serves for his moral improvement. But the number of men in the world is as nothing compared with that of all other sentinent beings, and these often suffer greatly without any moral improvement. A being so powerful and so full of knowledge as a God who could create the universe, is to our finite minds omnipotent and omniscient, and it revolts our understanding to supose that his benevolence is not unbounded, for what advantage can there be in the suffering of millions of the lower animals throughout almost endless time?
But then arises the doubt -- can the mind of man, which has, as I fully believe, been developed from a mind as low as the possessed by the lowest animal, be trusted when it draws such a grand conclusions? May not these be the result of the connection between cause and effect which strikes us as a necessary one, but probably depends merely on inherited experience? Nor must we overlook the probability of the constant inculcation in a belief in God on the minds of children producing so strong and perhaps an inherited effect on their brains not yet fully developed, that it would be as difficult for them to throw off their belief in God, as for a monkey to throw off its instinctive fear and hatred of a snake.
I cannot pretend to throw the least light on such abstruse problems. The mystery of the beginning of all things is insoluble to us; and I for one must be content to remain an Agnostic.
You know that feeling you get when you read something and it's as if some powerful ghost from the past is speaking directly to you, moving you, tickling your brain? That's how I feel when I read Darwin. Simply stirring!
Darwin on Religion