Although Voltaire denounced Christianity, he was a deist and believed in the existence of a creator-God throughout his life. This put him at odds with some of the other French Enlightenment figures, such as the hardline atheist Baron d'Holbach, who categorically rejected the existence of a creator-God in his magnum opus, the System of Nature (Système de la Nature, first published in 1770).

When asked if he had penned the System of Nature (which was published under a false name, like most atheist literature at the time), Voltaire denounced it, saying it was based on an "astonishing error." In the entry "Dieu--Dieux" ("God--Gods") in Questions sur l'Encyclopédie (1770-1771), he directly attacked Spinozism and d'Holbach, claiming that the System of Nature was ultimately founded on a 'story of the eels.' Voltaire was here referring to the work of John Needham, an English clergyman and member of the Royal Society, who had observed organisms with his microscope, tiny eels generating in grains of blighted wheat. D'Holbach had briefly referred to Needham's findings in the System of Nature, citing them as evidence that the generative powers of nature might be much greater than commonly supposed. Perhaps nature itself could be the cause of life and thus no creator-God needed to explain it.

Spontaneous generation at the hands of nature went very much against Voltaire's belief in preformation at the hands of a deity. The animal kingdom, which furnished abundant examples of divine craftsmanship, could not possibly, to his mind, be the product of processes in wheat flour. To produce intelligence without intelligence was patently absurd. Thus, in one of Voltaire's letters, written in 1770, the System of Nature is quite unfairly dismissed as being entirely founded on a "false experiment made by an Irish Jesuit who has been mistaken for a philosopher." Incidentally, Needham was neither Irish nor a Jesuit.

Sources:

Shirley A. Roe, "Voltaire Versus Needham: Atheism, Materialism and the Generation of Life" Journal of the History of Ideas, Vol. 46, No. 1, (Jan. - Mar., 1985), pp. 69-70.

Voltaire, Questions sur l'Encyclopédie, 2nd edition, Vol. 4. (London, 1771), "Dieu, Dieux," p. 264-268. For an English translation, see A Philosophical Dictionary from the French of M. de Voltaire, Vol. 3, second edition (London, 1824), "God--Gods," p. 322.