In 1954, Congress added “under God” to the previously secular Pledge of Allegiance. It placed a similar slogan, “In God We Trust,” on postage that year and voted the following year to add it to paper money; in 1956, it became the nation’s official motto.
During these years, Americans were told, time and time again, not just that the country should be a Christian nation, but that it always had been one. They soon came to think of the United States as “one nation under God.” They’ve believed it ever since.
Kevin M. Kruse, “How Business Made Us Christian”, The New York Times (15 March 2015), SR12.