Basically, Buckner discussed why the United States is not a Christian nation. He challenged other freethinkers to ask people who make this claim to explain what they mean by it. One could certainly argue that America has a Christian majority, but it also has a white majority, a female majority, and a majority of unmarried persons. If a nation is defined by its majority, then ours is a single white female Christian nation. You can easily understand why defining a nation by its majority does not always make sense. Sometimes when people claim that the US is a Christian nation, they mean that it was founded on Christian principles. Buckner asks these people to point to one single word in the Constitution that expresses any Biblical idea. Similarly, he challenges them to find one passage in the Bible that grants human beings civil rights; not divine rights and rewards, but the democratic ideas founded in the Constitution (such as the freedom of speech or the freedom of religion).
Buckner went on to explain that if we look at other historical documents of our past, then we can understand why we would not want to strictly adhere to every idea put forward upon our nation’s founding. In the Mayflower Compact, for example, the governing document of the Plymouth Colony, the settlers swore to remain “loyal subjects of [their] dread Sovereign Lord King James”, and idea that would later result in revolution.
Let’s not forget that past attempts to inject Christianity into the veins of our governing body have failed throughout history.
Founded in 1864 by a coalition of conservative Protestant ministers, the [National Reform Association’s] top goal was to add a "Christian nation" amendment to the U.S. Constitution. The ministers were convinced that the Civil War was God's vengeance on America for omitting religious language from the Constitution, and they sought to rectify that situation.
The House of Representatives rejected the amendment in 1874 and 1896. Recommending a vote against it in 1874, the House Judiciary Committee cited "the dangers which the union between church and state had imposed upon so many nations of the Old World...."
In 1950, a brief flurry of activity temporarily resuscitated the organization, at that time based in Topeka, when Congress considered yet another "Christian nation" amendment. The proposal, introduced by Sen. Ralph Flanders, a Vermont Republican, would have added language that "devoutly recognizes the Authority and Law of Jesus Christ, Saviour and Ruler of nations, through whom are bestowed the blessings of liberty."
The new "Christian nation" amendment was an even bigger bust than its predecessors, and it was never reported out of committee. Efforts to revive it in 1961, '63 and '65 were unsuccessful.
[From Americans United]