The Supreme Court's Establishment Clause cases are sometimes berated as exercises in unprincipled line-drawing based on the personal preferences of the justices, and of one recently retired justice in particular. There's something to this view. Some of the cases, however, leave the reader in a bewilderment too deep to be explained by mere lack of principle. In these cases, one really gets the feeling that the justices haven't grasped the nature of the thing in dispute--that they literally don't know what they're arguing about.
An article in the new issue of the Journal of the American Academy of Religion provides a religious-studies framework that could be a little help in some of these cases, if the Court were open to it. The article, The Pledge of Allegiance and the Meanings and Limits of Civil Religion, by Grace Y. Kao and Jerome E. Copulsky, analyzes the Pledge as a ritual of the nation's "civil religion," in the sense in which Robert Bellah used the term in his 1967 essay "Civil Religion in America." More particularly, Kao and Copulsky use "civil religion" to refer "to a symbolic system that binds members of a political community to one another through shared historical narratives, myths, rituals, and some notion of transcendence (e.g. the people, the nation, its overarching values and/or God.)" Civil religion, they say, functions to legitimate the social order, mobilize its members, and endorse institutions and authorities. They then consider the Pledge, and in particular the issue of the phrase "under God," in light of four modes of civil religion: preservationist, pluralist, priestly, and prophetic. By making sense of the Pledge as a whole they make make it possible to consider the controversial two words in a meaningful context.
The Court has never adopted the idea of civil religion, and perhaps it would be impractical for it to do so as such, if for no other reason than that the word "religion" makes state endorsement of it sound potentially unconstitutional. (Kao and Copulsky appear to assume implicitly that state sponsorship of civil religion in some forms, at least, is constitutional.) But unless and until the Court adopts Justice Thomas's view of the Establishment Clause (that it resists incorporation), the jurisprudence would be improved by a better effort to grapple with the meaning and purpose of the state-sponsored ceremonies and displays in which Establishment Clause controversies often arise. Kao & Copulsky's paper would be a good place to start.