People seem to think that it means that Western Civilization was developed by Jews and Christians, using values common to Judaism and Christianity. But it does not mean that at all. Jews and Judaism had almost nothing to do with the creation of Western Civilization.
“The attack on the Jew ... is an attack on Christianity itself and on the Judeo-Christian basis of our Western civilization.” San Francisco Chronicle (Oct. 9, 1947)So "Judeo-Christian" refers to obscure Christianized Jews, or to modern Jews like Ben Shapiro who are not on the Christian-hating left.
The notion that Western civilization rests on a Judeo-Christian basis is very largely an invention of the 1940s, when Jews felt a sudden and unprecedented desire to join the Western club and lock arms with their Christian “brothers.” Although a palpable oxymoron, the phrase prospered in the years that followed, and is now well established as one of the hardier weeds in the unlovely garden of American political cant.
Obviously, there can be no such thing as Judeo-Christian values when Jews specifically reject the highest Christian value, which is Christ. ...
Before 1940, historians used the phrase Judeo-Christian to describe sects of Christianized Jews, such as the Ebionites and Nazarenes, which briefly flourished in the early days of the faith. In his History of the Origins of Christianity (1888), for instance, Ernest Renan used the adjective to denote Jews who accepted Jesus as the Messiah, but who continued to observe the Mosaic Law and held themselves aloof from the gentile Church (in other words, these Judeo-Christians were the people Paul was really talking about in Galatians 3:28).
These Judeo-Christians also dissented from the emerging Trinitarian doctrine of the orthodox Church, denied that Jesus was divine, and seem to have taught that he was a prophet, an angel, or some sort of ghostly apparition. Renan suggests that they esteemed Jesus in much the same way as he would later be esteemed in Islam, which Renan called “a sort of Arab prolongation of Judeo Christianity.”
The Judeo-Christian sects were persecuted and destroyed by orthodox Jews, who abominated Jesus as the grossest of blasphemers, and who likely harried from their synagogues anyone who said he was anything less than a very wicked man.
A new book, The Founding Myth: Why Christian Nationalism Is Un-American, May 14, 2019, by Andrew L Seidel, argues:
Seidel, a constitutional attorney, provides a fervent takedown of Christian Nationalism in his furious debut. After support by far-right Christian nationalists helped Donald Trump win the U.S. presidency, Seidel worries that Evangelical political influence is increasing and dangerous. He argues that America was not founded as a Christian nation on Judeo-Christian principles, and thus Christian nationalists are inherently wrong. Judeo-Christian principles, he argues, are directly opposed to the Enlightenment principles on which the United States was founded: ‘to put it bluntly, Christianity is un-American.’I agree that America was not founded on Judeo-Christian principles. It makes more sense to say America was founded as a White supremacist nation.
Siedel says America was founded on "Enlightenment principles", but that is just another way of saying White Christian cultural values. The word Enlightenment just means the beliefs of White Christians about 250 years ago.
He particularly attacks "Christian Nationalism". Presumably his argument is based on Jefferson being a non-denominational Christian who did not want the state to establish any particular Christian denomination. And he was not what we would call today an evangelical. But he was still a Christian nationalist, and so were all the other founding fathers. What part of that can be doubted? He was certainly guided by Christian principles, he was certainly a nationalist, and he certainly lived in a nation of Christians.