Austen's reference is to a section of William Cowper's lengthy poem "Truth," with which many of her readers would likely have been familiar. Cowper's dig at Voltaire, who commented that "if God did not exist, it would be necessary to invent him," has often been misconstrued as evidence of Voltaire's atheism. In fact, he was a deist. See https://graceonlinelibrary.org/church-history/sermons-tracts/truth-by-william-cowper/