The Pledge of Allegiance of the United States of America was written by Francis Bellamy in 1892. The original form did not contain the words "under God". 

In 1954, Charles Oakman, a Republican Representative from Michigan, introduces a bill to incorporate "under God" into the Pledge of Allegiance. Congress passed the legislation it was signed into law by President Dwight D. Eisenhower. For more information, see Wikipedia.

This law clearly violates one of the most important parts of the United States Constitution, namely the First Amendment protection against the establishment of religion. In fact, it's unthinkable that such a law could exist in a country that was founded on the principal of separation of church and state.

One group which has taken up the cause is FreeThoughtAction. They have started a campaign to raise awareness by running billboard ads displaying "One Nation Indivisible", see below.

FreeThoughtAction also sells t-shirts, bumper stickers and mugs displaying "One Nation Indivisible". The group is run by volunteers and all proceeds go to financing awareness campaigns. You can make a tax-deductible donation by clicking on the link below.