According to Adam Kirsch in his recent review in Tablet Magazine, this is precisely the question that Kevin M. Schultz tries to answer in his book, Tri-Faith America: How Catholics and Jews Held America to Its Protestant Promise (Oxford). The answer Schultz supplies, according to Kirsch (I have not yet seen the book), is quite simple: “The change came about in the 1930s and 1940s, thanks primarily to the concerted effort of the National Conference of Christians and Jews, a lobbying and educational group founded in 1927.” Schultz tells the story of the NCCJ and its (largely successful) mission to forge a common language in America between Jews, Catholics, and Protestants. The argument is certainly plausible, particularly during World War II and the post-war period. It fits neatly into the narrative of the development of the multi-cultural melting pot that was America in the post-war period.
The dramatic increase in the use of “Judeo-Christian” seems to buck against the replacement in America of the image of the “melting pot” with that of “multiculturalism” or the “mosaic” as the governing metaphor in America of cultural relationships. (This is dramatically illustrated here.) Jews and Christians – all of them – are now lumped into one category, perhaps in recent years, as Kirsch might suggest (I am stretching his words here) to contrast America with Islamic civilization or the like – and this is before 9/11.