“The White House is the pulpit of the nation,” the American journalist James Reston once claimed, “and the president is its chaplain.” No wonder, then, that the inflammatory sermons of Barack Obama's own spiritual mentor are causing alarm.

Voters contemplating electing a black president are now also pondering whether they want a congregant of the Rev Jeremiah Wright's branch of the United Church of Christ. Would not the Methodist Hillary Clinton or Episcopalian-turned-Baptist John McCain be a bit more mainstream?

Despite the constitutional separation of Church and State, the one category of American that seems ruled out for the White House is not black or female, but atheist.

Yet it is a relatively recent assumption that the president should be a regular churchgoer. Although none renounced religion, 11 of the 43 presidents were not members of a church. For most of the presidency's history, a candidate's spiritual affiliation was rarely an issue unless he was a practising Catholic.

Many of the early presidents were far more freethinking than their official Episcopalian designation suggested. That George Washington believed in his Maker is not disputed. It is what he thought beyond this that divides historians.

Some, like the Constitution's architect, James Madison, kept their religious sensibilities private. The Unitarians, John Adams and John Quincy Adams, denied that Jesus was necessarily the Son of God. The author of the Declaration of Independence, Thomas Jefferson, concluded: “I am of a sect by myself, as far as I know.”

Steeped in biblical language, Abraham Lincoln mentioned God in his Gettysburg Address. The nature of his faith, however, remains debatable. Both his widow and his long-term friend, Judge David Davis, maintained “he had no faith in the Christian sense of term”.

It was with the inauguration of Dwight D. Eisenhower that it became normal for presidents to extol the depth of their beliefs. Raised among a sect that was a forerunner to the Jehovah's Witnesses, Eisenhower became a Presbyterian. In 1954 he signed into law the addition of the words “under God” into the previously non-religious Pledge of Allegiance. Two years later, he approved “In God We Trust” as the USA's motto.

These changes were in keeping with their times. The evangelical revival not only created a constituency for the Republicans. Democrats such as Jimmy Carter also campaigned on being “born again”.

For all this, presidents get only so much latitude. As the 1968 presidential hopeful Senator Eugene McCarthy pointed out, in Washington “only two kinds of religion are tolerated: vague beliefs strongly affirmed and strong beliefs vaguely expressed”.

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