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Richard Nixon identified himself as "a church-going Christian," but he was "one of the most corrupt and paranoid men to occupy the Oval Office," CNN's John Blake says.

A recent Gallup poll found that 18 percent of Americans would not vote for an otherwise qualified Mormon candidate for president. Meanwhile, Jane Pitt, primarily known for being Brad Pitt's mother, rippled the political endorsement waters when she wrote that "any Christian should spend much time in prayer before refusing to vote for a family man with high morals, business experience, who is against abortion and shares Christian conviction concerning homosexuality just because he is a Mormon."

It can be confusing trying to determine in advance if Americans will vote for a Mormon presidential candidate. But according to CNN's John Blake, a candidate's religious preference really shouldn't matter because "piety and presidential performance don't always match."

"Some of America's more religious presidents have been its most brutal," Blake writes. "And two of its greatest presidents wouldn't even be considered Christians today, scholars say."

For example, Niels C. Nielsen, author of "God in the Obama Era," says that Abraham Lincoln never joined a church and was not a Christian.

"Lincoln believed in an active God, he believed in providence," Nielsen said. "But if you asked Lincoln if he believed in the deity of Jesus, he would have said no."

Similarly, George Washington was most likely a Deist, Blake said, or "someone who believed in a divine, beneficent being who ordered the world." He cites historian Darrin Grinder, author of "The Presidents and Their Faith," who says that while clergy would often try to get Washington to profess Christianity, he refused to do so.

At the other end of the spectrum, according to Blake, is Richard Nixon, who referred to himself as a "life-long Quaker and a church-going Christian" who played piano in church, taught Sunday school and praised Jesus at revivals. Blake said Nixon turned out to be "one of the most corrupt and paranoid men to occupy the Oval Office."

Drawing from various authors and experts, Blake examines the religious beliefs of a number of U.S. presidents, including what he calls the "unconventional religious background" of President Barack Obama. He quotes Grinder, who says the public interest in the religious background of presidential candidates has continued through the years because "we still want our presidents to act like a politician and a priest."

"The religious rhetoric gets louder each year," Grinder says. "That's not going to change anytime soon."