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But the 31-word passage has evolved over time. Most people don't realize the phrase "under God" wasn't included in Flag Day in 1954 - 60 years ago today.
Here's what went down.
In 1892, Francis Bellamy, a minister from upstate New York, reportedly wrote the pledge as an expression of fealty to the U.S. It read: "I pledge allegiance to my flag and the republic for which it stands, one nation, indivisible, with liberty and justice for all." Some consider Bellamy a socialist and his creation a criticism of rampant greed and hyper-individuality.
Over the next 50 years or so, Congress would eventually add "of the United States" after "flag" and a simple "to" before "republic." It wasn't too controversial.
Then, an attorney from Illinois, Louis Bowman, shook the wording up a bit. At a meeting of the Sons of the American Revolution in 1948, he added "under God," claiming Abraham Lincoln used the same phrase in his Gettysburg Address. Almost all reported transcripts from the speech do include "that the nation shall, under God, have a new birth of freedom."
Bowman continued to deliver his version of the Pledge, and others, like the Knights of Columbus, began reciting it, too. Various people even wrote letters to the president at the time, Harry Truman, and met with him to request the more religious tone.
Finally, the government became involved. In 1953, Louis Rabaut, a democrat from Michigan sponsored a resolution to add the words "under God" to the Pledge. By then, the decision was up to President Dwight D. Eisenhower. Recently baptized as a Presbyterian, he heard a sermon, arguing the words "under God" from Lincoln's speech set the United States apart from others as a nation. The Cold War was gaining steam, and Eisenhower was fighting communism from all parts of the globe.
The next day, the president encouraged Charles Oakman, a republican also from Michigan, to re-introduce the bill, which Congress passed. Eisenhower signed it into law on June 14, 1954. A story announcing the news in the Washington Post quoted him as saying the new version would add "spiritual weapons which will forever be our country's most powerful resource."
Naturally, in a nation with growing diversity of religions, "under God" has proven a polarizing phrase. Separation of church and state also factors into the politicized discussion.
Two years later, on Flag Day again, Eisenhower also made "In God We Trust" our nation's official motto. The man must have loved his new religion.