I would like to make the arguments in the upcoming Supreme Court case of Unified School District v. Newdow but realistically I don't have the time and money to afford to file a brief. If I were to file - here's some of the arguments I would make.
The Pledge is a law. It is specified in federal code. It is an act of patriotism. "One Nation Under God" is a religious statement and with that statement there it marries patriotism with religion. It forces non-believers who whish to recite the pledge to make a religious affirmation that they don't believe in. The pledge is a statement of national identity. Its much more than a ceremonial or historic thing. It is more like an oath.
The Pledge is distinguished from other less important acts that include the mention of God. It is not merely ceremonial or historic. It is codified into federal law.
Lets look at mathematical sets. The big set is all people of all religions. This can be divided into to subsets - monotheists who believe in exactly one god - and everyone else who believe in multiple gods or no god. The argument is perhaps that there are so many monotheists that we can agree that this is a monotheistic country.
The flaw is that it excludes a class of people - non-monotheists. If it said "Under Jesus" or "Under Allah" or "Under Zeus" or "Under the Sacred Tree of Knowledge" then it would be more exclusive. They would argue that "Under God" is a sufficiently large subset to be close enough to be compliant. But because it is part of the phrase "One Nation - Under God" it ties our national identity to a religious affirmation.
Basically - to have "One Nation Under God" - in the context of the Pledge - is to have government make a determination that god exists. I would ask the court to be required to rule that god exists in order to include it in the Pledge. If the Pledge said "Under Peter Pan" then that would be right because Peter Pan is a fictional character and it would be wrong for the government to include a fictional character in the pledge laws.
God is a fiction character. He is no more real that Peter Pan. There is no logical difference between God and Peter Pan. Now - the court might not agree with that - but that brings up the real issue.
It is not up to the court or the government to determine if God exists. The first amendment prohibits the government from determining religious questions. Religious questions are for Religions to determine. It is up to Marc Perkel, Ferry Falwell, the Pope, the Dalai Lama, and individual citizens to decide that question. Judges and governments do not determine religious issues. It is not their place to do so. It is up to the people - not the government to decide whether or not to believe in God and that decision must be a separate act from pledging one allegiance to the nation.
The First Amendment of the Constitution "Establishment Clause" prohibits the government from establishing religion. Although the court recognized an exception for acts that have become the "fabric of society" in this case tying the pledge to a religious affirmation is designed to alter the fabric of society for the purpose of including government sponsored religious indoctrination to support infecting the fabric of society with the belief in the fictional character God. Including the phrase "One Nation Under God" establishes a government preference for monotheism over other religions.
Those who support the phrase "Under God" argue that is is a recognition of our national history, that the forefathers believed in God and referenced him in important documents. This is all part of the "fabric of society" argument that becuse we are all infected we should just accept the infection and declare we are a nation of people infected with a false belief and keep the infection in society.
As the founder of the Church of Reality I find this argument particulary offensive because the "fabric of society" argument basically say we are required to live the lie and to perpetuate the lie in the Tree of Knowledge. This precludes our Principle of Positive Evolution and the Principle of Bullshit and the very foundation of our church doctrine. To require our children to recite falsehood in order to make a patriotic statement is oppressive.