The words "under God" were added to the US Pledge of Allegiance in 1954, during the Cold War, under the Eisenhower administration, to mark a distinction between the USA and so-called 'godless' communism of the Soviet Union and the People's Republic of China.

When people today quote "one nation, under God" as evidence that the founders of this country created a Christian Nationalist country, they are repeating historical inaccuracies. You are welcome to correct them.

@VisualStuart Also, the whole Prayer to the Flag has nothing to do with the Founders; it only dates to the 20s. As a historian, I'm certain the Founders would have been outraged. Washington in particular couldn't have missed its monarchist overtones -- his particular trigger -- and would promptly have unleashed his famous acerbic contempt on the whole project.
@RustyRing @VisualStuart I suspect Jefferson would have had a word or two against it as well.
@Tattered @VisualStuart Frankly, given its fatuous nature, I suspect that even John Adams - a man with proven monarchist reflexes -- would have passed on it. ("Flag? Really?") It's just not about America.