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.
...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.In Elk Grove Unified School District v. Newdow, 542 U.S. 1 (2004).
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").
Used for the purpose of defending religious references by the government as harmless, he says the term "ceremonial deism" is grossly inaccurate and even dangerous.