David Greenberg is associate professor of history and of journalism and media studies at Rutgers University and the author of several works of political history, including "Nixon's Shadow: The History of an Image."

We Americans pride ourselves on our religious pluralism and toleration. Although presidents do feel obliged to end every speech with the title of an Irving Berlin song ("God Bless America"), by and large they adhere to the Founding Fathers' ideal of separation of church and state. But contrary to this general rule there each year arises the exceptional custom of White House Christmas cards.

Should the president and first lady really be issuing messages to celebrate a religious holiday that not all Americans celebrate? Strictly speaking, probably not, even if the costs are picked up by the political parties. Yet the practice has never incurred the wrath of the American Civil Liberties Union. That's probably because, since the beginning, these messages have usually taken on an inclusive, if not bland, character — one that manages to respect the holiday season and simultaneously to give scant offense.

According to Mary Evans Seeley's "Season's Greetings from the White House, " the key work on White House Christmas celebrations, presidential holiday messages originated with Calvin Coolidge. In 1923, Coolidge's first winter in office, Middlebury College, in his home state of Vermont, donated a 60-foot fir tree that was installed on the Ellipse, south of the Treasury Building, and illuminated in a public ceremony. In subsequent years, Coolidge — an unsung pioneer in the use of radio and mass media — became not only the first president to light a Christmas tree in public but also the first one to deliver a Christmas message over the radio and the first to issue a written statement, which many newspapers across the country reprinted.

Although issued on a Christian holiday, Coolidge's statement was, notably, mainly secular in nature. The 1920s witnessed cultural wars as fierce as those that have racked the country since the 1960s — over immigration (even then), Prohibition, the Ku Klux Klan — and Coolidge, though a conservative Republican who was a pious Christian in private, sought to maintain an ecumenical tone. Although the vague reference to "a Savior" gave his message a mild Christian cast, what the president called for was not any specific religious belief but "a state of mind" that cherished "peace and goodwill."...