"In God We Trust" first appeared on U.S. coins during the Civil War in 1864. Nearly a hundred years later, in 1956, an act of Congress made it the national motto. It began appearing on paper currency the following year, in 1957. So it's ascendancy is directly related to two wars, one hot, one cold, but both desperate. Perhaps "There are no atheists in foxholes" didn't have the right ring to it?
"E pluribus unum," on the other hand, was never the official motto, but it did appear on the official seal of the United States, and has since 1782. Interestingly, it's inclusion on the official seal emerged from a recommendation by a committee of Founding Fathers, who had gathered in 1776 no less to craft the symbols of the new nation.
So if we're going by strict constructionism, then, "E pluribus unum" wins the national motto game hands down. But of course our legal body is a flexible thing, changing with the times. So we've got "In God We Trust."
But "E pluribus unum" makes more sense in another historical way too. Most of the Founding Fathers were Christians of some sort or another, and most believed that a religion of future rewards and punishments was a vital part of 18th-century statecraft, but they almost to a man believed that there were more types of religion in the United States than could be described and upheld by a national body. After all, when Ben Franklin asked for a moment of prayer to help ease a particularly thorny debate in the Constitutional Convention, only three or four Founding Fathers thought it was a good idea. Instead, we got the First Amendment.
Perhaps last week's congressional vote to reaffirm the national motto as "In God We Trust" is just another reminder that those trying to prove America's distinct Christian heritage and mission are more interested in winning for souls for Jesus than in understanding the past. Or maybe the War on Christmas really is a real thing. Indeed, there are no atheists in the North Pole.