That would be from the 1954 revision of the Pledge of Alligence, just a tad later than the US Constitution which is the founding document of this country.<quoted text>
We are one nation under god.
The Pledge of Allegiance was written in 1892 by Francis Bellamy (1855–1931), who was a Baptist minister, a Christian socialist, and the cousin of socialist utopian novelist Edward Bellamy (1850–1898).
Prior to February 1954, no endeavor to get the Pledge officially amended succeeded. The final successful push came from George MacPherson Docherty. Some American presidents honored Lincoln's birthday by attending services at the church Lincoln attended, New York Avenue Presbyterian Church by sitting in Lincoln's pew on the Sunday nearest February 12. On February 7, 1954, with President Eisenhower sitting in Lincoln's pew, the church's pastor, George MacPherson Docherty, delivered a sermon based on the Gettysburg Address titled "A New Birth of Freedom." He argued that the nation's might lay not in arms but its spirit and higher purpose. He noted that the Pledge's sentiments could be those of any nation, that "there was something missing in the pledge, and that which was missing was the characteristic and definitive factor in the American way of life." He cited Lincoln's words "under God" as defining words that set the United States apart from other nations.
President Eisenhower had been baptized a Presbyterian very recently, just a year before. He responded enthusiastically to Docherty in a conversation following the service. Eisenhower acted on his suggestion the next day and on February 8, 1954, Rep. Charles Oakman (R-Mich.), introduced a bill to that effect. Congress passed the necessary legislation and Eisenhower signed the bill into law on Flag Day, June 14, 1954. Eisenhower stated "From this day forward, the millions of our school children will daily proclaim in every city and town, every village and rural school house, the dedication of our nation and our people to the Almighty.... In this way we are reaffirming the transcendence of religious faith in America's heritage and future; in this way we shall constantly strengthen those spiritual weapons which forever will be our country's most powerful resource, in peace or in war."
The phrase "under God" was incorporated into the Pledge of Allegiance June 14, 1954, by a Joint Resolution of Congress amending §4 of the Flag Code enacted in 1942.
...One objection states that a democratic republic built on freedom of dissent should not require its citizens to pledge allegiance to it, and that the First Amendment to the United States Constitution protects one's right to refrain from speaking or standing (also a form of speech). Another objection lies in the fact that the people who are most likely to recite the Pledge every day, small children in schools, cannot really give their consent or even completely understand the Pledge they are taking.