Is "Under God" Unconstitutional?

The U.S. 9th Circuit Court of Appeals is standing by its decision that reciting the 1954 version of the Pledge of Allegiance, which includes the words "one nation under God," is a violation of the Constitution's establishment clause.

Although the court and its supporters offer a number of arguments for their position, their case rests on the following general argument:

1. The Constitution forbids congress from passing any "law respecting an establishment of religion."
   
2. Congress added the words "under God" to the Pledge in 1954.
   
3. Although not mandatory, the Pledge is recited at many public events and in public schools.
   
4. The phrase "one nation under God" is a profession of religious belief, namely, a belief in monotheism.
   
5. "God" is an inherently religious concept.
   
6. Therefore, the 1954 Pledge is unconstitutional.

Where does this argument go wrong? At premises (4) and (5). Here's what the court wrote in Newdow vs. US Congress:

To recite the Pledge is not to describe the United States; instead, it is to swear allegiance to the values for which the flag stands: unity, indivisibility, liberty, justice, and--since 1954--monotheism. The text of the official Pledge, codified in federal law, impermissibly takes a position with respect to the purely religious question of the existence and identity of God. [Note]

The problem is that belief in God is neither a necessary nor a sufficient characteristic of religion. There are religions that do not profess belief in God, e.g., Theravada Buddhism and Taoism; and there are people who believe in God and reject religion, e.g., Benjamin Franklin. Monotheism is a philosophical doctrine that endorses the proposition "One God exists." Some religions endorse that proposition, and others do not; and some people endorse it but reject religion. Unfortunately, the court does offer any support for (5), instead taking the religiosity of the concept of God as a foregone conclusion, as do Michael Newdow, who brought the suit, and Rev. Barry Levin, executive director of Americans United for Separation of Church and State. What the court does offer is a tepid analogy:

A profession that we are a nation "under God" is identical, for Establishment Clause purposes, to a profession that we are a nation "under Jesus," a nation "under Vishnu," a nation "under Zeus," or a nation "under no god," because none of these professions can be neutral with respect to religion.

"One nation under God" is no more identical to "one nation under Jesus" or "one nation under Vishnu" than "I believe in God" is identical to "I believe in Jesus" or "I believe in Vishnu." To be fair, the court tries to justify this analogy not on the ground that these statements are conceptually identical (which is patently absurd), but that they are identical in terms of their neutrality with respect to religion, which is to say that they are not neutral. Nonetheless, to reiterate, monotheism is neutral with respect to religion; a religion can either take or leave monotheism, and a monotheist can either take or leave religion.

Since (5) is false, (4) is likewise false; and so the conclusion fails as well.

[Note] The quotes from Newdow vs. US Congress come from an Adobe Acrobat file on the opinions page of the U.S. 9th Circuit Court of Appeals. Search for "Newdow" to find the link to download the file.