To have so many candidates in the current campaign say faith is a significant part of their lives and has a major impact on how they think about politics and policies—this is unique." So says Gary Scott Smith, professor of history at Grove City College in Pennsylvania. He should know. He wrote a book that examined the faith of 11 American presidents (Faith and the Presidency from George Washington to George W. Bush, Oxford University Press), and is working on another that looks at the faith of 11 more, including President Obama.

Presidents have often made assertions of faith, Smith says, but to have such a high percentage of candidates do so is unusual in American history. Senior...