The First Amendment says "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; ..." The Pledge does not respect any establishment of religion. If it did, what establishment is it? The Roman Catholics? The Presbyterians? The Jews? The Pledge was purely optional and voluntary. If the Pledge is religious (and I don't think it is), then voluntarily saying it is the free exercise of religion.
The word God in the Pledge, on US coins and currency, and in the Declaration of Independence is a term of art. It refers to a political philosophy held by the US Founders that rights under the law were based on natural principles, and not derived from the grant of a king or from simple majority rule. Apparently the US Congress wanted to emphasize this point when it inserted "under God" in 1954, so as to distinguish our form of government from the ideologies that underlie Communism.
The 9th Circuit compares “under God” to “under Jesus”, and says that the Pledge is an endorsement of monotheism. It is not. Most of those who say it are Christians who believe in the three Gods of the Holy Trinity. Many are Catholics who believe in the Virgin Mary as well. Many of the Founders, such as Thomas Jefferson, were deists who did not even believe in a Christian God.
President Bush often ends his speeches with “God bless America.” Eg, he says it in this letter to children. If the Pledge is unconstitutional, then so is listening to the President. The vast majority of atheists do not take any offense to this, because it is a nondenominational and innocuous statement.
Even physicists, who are mostly atheists, often use the word God to refer to the laws of nature and similar concepts. Eg, people like Stephen Hawking use the word God, and no one is offended or thinks that he is making a religious statement. It is just a figure of speech.
The US Supreme Court will surely overrule Reinhardt and the 9th Circuit, as it usually does.