The Red Scare concerning communism.
"E pluribus unum" (out of many, one) was essentially considered the US motto until 1956, when Congress passed an act adopting "In God We Trust" as the country's official motto. Opposition to communism as a godless, atheistic ideology (and fear of its influence) was a motivator for the change. Similarly, the phrase "under God" was added to the Pledge of Allegiance by an act of Congress in 1954. Both acts were signed into law by President Dwight Eisenhower.