Voltaire, like many others before and after him, was awed by the order and the beauty of the universe, which he thought pointed to a supreme designer, just as a watch points to a watchmaker. In 1779, a year after Voltaire died, that idea was attacked by David Hume, a cheerful Scottish historian and philosopher, whose way of undermining religion was as arresting for its strategy as it was for its detail. Hume couldn’t have been more different from today’s militant atheists.
In his “Dialogues Concerning Natural Religion,” which was published posthumously, and reports imaginary discussions among three men, Hume prized apart the supposed analogy between the natural world and a designed artifact. Even if the analogy were apt, he pointed out, the most one could infer from it would be a superior craftsman, not an omnipotent and perfect deity. And, he argued, if it is necessary to ask who made the world it must also be necessary to ask who, or what, made that maker. In other words, God is merely the answer that you get if you do not ask enough questions. From the accounts of his friends, his letters, and some posthumous essays, it is clear that Hume had no trace of religion, did not believe in an afterlife, and was particularly disdainful of Christianity. He had a horror of zealotry. Yet his many writings on religion have a genial and even superficially pious tone. He wanted to convince his religious readers, and recognized that only gentle and reassuring persuasion would work. In a telling passage in the “Dialogues,” Hume has one of his characters remark that a person who openly proclaimed atheism, being guilty of “indiscretion and imprudence,” would not be very formidable.
It's a bit more complicated than this. We do know that Hume did not believe in an afterlife; and that he was disdainful of Christianity as an institution, particularly Scottish Presbyterianism. Near the end of his life he joked about not being able to complete his great work of ending the Christian superstition in Britain. But we also know that he represented himself to others as a theist and once said he didn't believe atheists existed because he had never met one (we know he said that because he said it at a dinner party full of atheists hosted by d'Holbach and Diderot noted it for posterity). He regularly contrasts true religion (good) with false religion (bad, if you hadn't guessed), and criticizes atheists not merely for "indiscretion and imprudence" (which phrase is actually attributed to Bacon, although the skeptic in the Dialogues explicitly claims to agree with it); but also has the same character remark that atheists can only be nominally so "and can never possibly be in earnest." We don't, in fact, know Hume's precise view of the matter; judgment of that point requires making guesses about how far his irony extends, and elaborate suppositions about the end he has in view in saying such things. All the limited evidence we have points to Hume's classifying himself as a theist (and none whatsoever to his classifying himself as an atheist); but all the evidence we have is scattered and difficult to place in a larger context, leaving obscure precisely what he took that to mean.
Likewise, the attack on the design argument, mentioned in the first paragraph, is considerably more ambiguous than it sounds; the last Part of the Dialogues contains a definite -- albeit convoluted, ambiguous, and difficult to interpret precisely -- affirmation of the design argument. Hume very explicitly and deliberately has the design argument survive, in some form, the attack made on it, even going so far as to have the skeptic deny that atheists can seriously reject its basic point. I have my own interpretation of this move, which will come out in the series of posts I'm doing on the subject. But there are quite a few complications in understanding Hume on this point, and no completely adequate interpretation has ever been found. (Good news for us Hume scholars, since it means there's plenty to uncover yet!)
UPDATE: Expanded a few overly concise points and corrected some of the more obscure phrasing.