The two core myths that purport to exist in symbiosis with one another but rather stand in dichotomy to each is that the United States is both founded upon as a Christian Nation and that America is a pluralistic nation due to a clear separation of Church and State. The two stand in contrast, however, as to acknowledge one religion as being superior would alienate all others that did not conform to it. Moreover, by the end of the American Revolution, an accouterment of religious splits had occurred. Though still Christian by any modern definition, the differences between Quakers, Lutherans, Baptists, and others were so great at that time that no one division could be considered THE religion. Immigration movements from China, Latin America, the Middle East, and Europe have brought scores of different religions; some of which still fall under the Judeo-Christian umbrella (Islam, Judaism, Catholicism), and others that share a diametrically opposed worldview from contemporary Western religion (Buddhism, Shintoism, Taoism, Hinduism). To say back at the dawn of the country that the United States was a Christian Nation would have been a non-truth, but in modern times there is not an argument to be had in that assertion. The United States is the most diverse homogeneity of competing ethos.
To look at the origins of this myth you have to go back... way back. Before the United States, before the colonies, before Europe even. The genesis of the West lies in the mechanics of Rome. Europe and the modern era credit Roman technology, democracy, and ingenuity as their zenith and example to us. But it's in the shadow archetypes of the Roman method where the stain of religious superiority bleeds through the veneer of austerity and goodwill.
The Romans were masters of pageantry. They kept immaculate records wherever they went and with the soldiers came the anthropologist-like scribes, known also as the missionaries. The Romans learned the ways of the people they conquered and through supplanting Roman pensioners into conquered land, establishing commerce, creating political hierarchies, they created a default Roman identity.
In the manner of conversion, the grandchildren grew so accommodated to tribal defeat and the passing away of tribal memory that they probably articulated it as a new possibility, as deliverance from tribal darkness, as freedom. In the first generation, it was trauma and Armageddon, in the second coping and surviving. In the third, the same thing was a new God (Jenkinson, 2018).
This method of conversion through land acquisition would become a model for future colonialism and American Western expansion, known as Manifest Destiny. Making pagans out of indigenous people, they would convert the people by converting the land. In the waning days of the Roman Empire, the Church stood as the arbiter of the good news during what would come to be known as the Dark Ages. Seen as their moral and religious edict and using Roman tactics, they went to the villages, establishing churches and communities and laws, and with that sought to obliterate ancestor-worship, animism, and anything contrary to the narrative of the one true God.
This generational institution of superiority and piety informed the people living under its reach. In the 2006 movie Rocky Balboa, Sylvester Stallone’s character shares with his adult son a piece of wisdom that is well worth its salt: "If you stay in a place long enough, you become that place." The people who came to the Americas were disenchanted with religious and political oppression, but to say they weren't informed by the moral understanding of the time falls short. The people came and with them came the religious interpretations that, though at the time were distinct from whence they came, still followed the thread of conversion through disavowing of past beliefs. That thread of thought allowed for the institution of boarding schools for Native Americans, which sought to "kill the Indian, and save the man." It informed Supreme Court decisions to allow the destruction of sacred lands (Adams et al., 2018), and its informed xenophobia in modern times, from the mass internment of Japanese Americans, many of which were Buddhist, in WWII, to the travel bans implemented during the Trump administration.