Ceremonial deism is a legal term used in the United States for nominally religious statements and practices deemed to be merely ritual and non-religious through long customary usage. Proposed examples of ceremonial deism include the reference to God introduced into the Pledge of Allegiance in 1954, and the phrase "In God We Trust" on U.S. currency.
The term was coined in 1962 by the then-dean of Yale Law School, Eugene Rostow, and has been used since 1984 by the Supreme Court of the United States to assess exemptions from the Establishment Clause of the First Amendment to the U.S. Constitution.
Usage by the Supreme Court
...I would suggest that such practices as the designation of "In God We Trust" as our national motto, or the references to God contained in the Pledge of Allegiance to the flag can best be understood, in Dean Rostow's apt phrase, as a form a "ceremonial deism," protected from Establishment Clause scrutiny chiefly because they have lost through rote repetition any significant religious content. [emphasis added, citations omitted]
In Elk Grove Unified School District v. Newdow, 542 U.S. 1 (2004). Justice O'Connor, concurring in the opinion, invoked the term in her analysis of the nature of the phrase "under God" in the Pledge of Allegiance, saying in part
There are no de minimis violations of the Constitution – no constitutional harms so slight that the courts are obliged to ignore them. Given the values that the Establishment Clause was meant to serve, however, I believe that government can, in a discrete category of cases, acknowledge or refer to the divine without offending the Constitution. This category of "ceremonial deism" most clearly encompasses such things as the national motto ("In God We Trust"), religious references in traditional patriotic songs such as "The Star-Spangled Banner", and the words with which the Marshal of this Court opens each of its sessions ("God save the United States and this honorable Court"). These references are not minor trespasses upon the Establishment Clause to which I turn a blind eye. Instead, their history, character, and context prevent them from being constitutional violations at all. [emphasis added, citations omitted]
Professor Martha Nussbaum at the University of Chicago Law school stated in 2008 that "'Ceremonial Deism' is an odd name for a ritual affirmation that a Deist would be very reluctant to endorse, since Deists think of God as a rational causal principle but not as a personal judge and father."