The human impulse to categorize the world is a way to logically compartmentalize the similarities and differences among objects and subjects. This impulse is not always accurate; a desire to typify humans involved inadequate concepts such as the classical Greek elements of water, fire, air and earth, the medieval humors of blood, yellow bile, black bile and phlegm, the timeless practice of astrology, the pseudoscience of phrenology and the muddied waters of racial purity. In the face of the maturation of science and subsequent establishment of facts, this impulse remains constant even as the methods change, such as the modern categorizations of the political spectrum and the Myers-Briggs test. This impulse of categorization is not just fixated on humans; we fit institutions, traditions and beliefs into categories to better understand and judge. The most important of these may be the concept of "the West".
"The West" is an odd term to use if you lack context. After all, every place is west of somewhere when you're on a sphere. The part of the world we call "the West" gets its name from its relative orientation to the rest of the Eurasian landmass - Europe is a bite-size bit on a giant stretch of land too massive for somebody in the classical era to comprehend. Humans in those archaic days were just as conscious of group differences as we are today, and the philosophizing, bickering Greek city-states were notably different from the multicultural, sprawling Persian Empire. While they were by no means isolated from each other, this distinction was enough of a rallying cry to back up the sacrifice of the Spartans (et al) at Thermopylae. That timeless suicide mission can be dramatized as the birth of "the West", but what made "the West" truly special came much later.
The application of religion to help maintain state control is a legacy of authority stretching back to the earliest writings and artifacts. The Romans used it throughout their history; when the Western Roman Empire collapsed it was priests and monks who became the torchbearers for knowledge and authority. Ascendant warlords found greater benefit in applying their knowledge than pillaging their relics, and a system of mutual protection took hold - warlords protecting the holy houses from raiders, while priests protected the warlords realm from inefficiency and illiteracy. Priests also brought the elusive and powerful blessing of God - always a benefit when trying to overawe your subjects and enemies. As feudal holdings coalesced, the priesthood became a faction as necessary for ruling as the nobility or the burghers, and Christianity in "the West" became politicized.
This politicization was made easier with the vast amount of wealth that holy houses were able to accumulate over time. This enormous wealth funded the majestic cultural heirlooms that tourists gaze at in Europe - statues, paintings, frescos, et al - were invariably drawn from the tithes burdening peasants inside the priests' demesne or from pillaging heretics and heathens. But the thing about money is that there is never enough, and the increasing amount of wealth being squeezed out of peasants - most notably the selling of indulgences, simplified as buying a get-out-of-hell card - led to the Protestant Reformation. The Reformation led to the Counter-Reformation, and as Christian identity and the personal relationship with God was up for debate (and for battle), culture began to evolve in the absence of absolute spiritual authority.
This was no minor thing. Monarchs overwhelmingly often implied their authority as coming from God, and to imply their spiritual conduit breaking was not only heresy, it was rebellion. While some monarchs doubled down on their religious relationship to Rome, others took a more independent measure. Foremost is Henry VIII of England. Best known for going through wives faster than the Mets tear through promising young pitchers, Henry had a church problem. He saw the church as being more a barrier to his power than a supporter for it, and his break from Rome and foundation of the Anglican Church would open up Britain to centuries of religious conflict. What's more, his Dissolution of the Monasteries destroyed a staple of British identity that reached back to Roman times, helped convert the Germans, and resisted the Vikings. As "the West" in general and Britain in particular passionately flailed about for religious meaning and guidance, religious separatists broke off of established churches, each group declaring their path the proper way to follow a Godly life.
One of these was the Brownists. A Puritan sect with strong Calvinist leanings, the Brownists were separatists from the Anglican Church who refused to follow laws that they considered oppressive and heretical. First attempting to find refuge in the Netherlands, their trek across the Atlantic to land at Plymouth Rock in Massachusetts is generally seen as the foundational event in American history. This is, as most myths, inaccurate. It wasn't even the first British settlement in the future United States - Jamestown precedes it by more than a decade. However, it was indubitably the Pilgrim's faith in their particular strain of Christianity that propelled them across the Atlantic, and the foundation of Massachusetts is indelibly linked to this religious sect.
But though Massachusetts may have had a specific religious foundation, this does not carry over to the other states. Pennsylvania was founded with having religious freedom in mind, rather than religious governance, and Maryland was a haven for Catholics. When the United States came into existence, these religious leanings were very much evident among the individual states. The religious differences among the former colonies is a direct result of the Reformation, and to avoid the religious wars that had wracked Europe for hundreds of years, the Founders decided to avoid enshrining one strain of Christianity as the official religion by making religious freedom a cornerstone right of the republic.
One would imagine that it's easy to describe Christianity in a coherent and total manner, but the simple matter of fact is that there have been so may heresies, ideas, revisions, and reformations that the only constant is that a guy named Jesus is involved somewhere. From the Taipings believing their leader was the younger Chinese brother of Jesus to the Arians stating that Jesus was not divine to Mormons believing Jesus traveled to America, Christianity has been molded to meet the needs of its followers as much as it molds its followers to be more "Christian" - whatever that may entail. And with so many heresies abound, a question must be asked - when somebody claims that America is a Christian nation, which Christianity are they talking about?
We have straightforward answers for individual states such as Puritan Massachusetts and Mormon Utah, but for the United States of America, there is no single strain of Christianity to bind it together. Instead, the principles that the United States of America was founded on was the Enlightenment values of personal liberty and self-determination of faith - a lesson learned from royal and papal attempts to control religion instigating wars of extraordinary length and destruction in Europe. Those that believe that America is a Christian nation will point toward the Enlightenment being born out of Christianity, but this is quirk of our need to categorize - how much of the Enlightenment is Christian?
There is a mental exercise known as the Ship of Theseus, which I will modernize by relabeling it the Plane of Yeager. Let's say you came into ownership of the plane that Chuck Yeager used to punch through the sound barrier, and for kicks you decide to fly it around. Over time, the parts of the plane wear down and need to be replaced. How many parts can you replace and still be able to call it Yeager's plane? Similarly, how much can you change Christianity and still call it Christianity? Is Mormonism a Christian religion if it believes that their most important prophet was Joseph Smith? Is it more Christian than Islam, which also believes Jesus to be a prophet, but lesser than Mohammed? The humanistic belief structure that grew out of the Enlightenment was a revival of interest in Greek philosophy that allowed people to address faith in a logical manner. This is similar to how the Mutazilites tried to rationalize Islam before they lost the ideological war to Ashirites, and we find ourselves locked in a similar battle in America today.
We live in an America where a substantial amount of voters hold the Bible in higher regard than the Constitution. The idea of America as a Christian nation is, for them, unquestionable. Deviation from their interpretation of their scripture is heresy that weakens that nation and allows evil to enter their homes and hearts. These people have been a part of America since its foundation and inherently pose no danger - the danger that exists is because the political structure of this country allows them to dictate policy issues for half of the nation. And the influence of people who may be perfectly alright with the Apocalypse happening because they get to see Jesus faster and the wicked get what's coming to them can not be disregarded.
We can not and should not forget our history, and the history of "the West" is forever intertwined with Christianity. But this does not shackle us to the past - many events happened between the Crucifixion and the foundation of the United States, and the Enlightenment that gave America the values that makes it unique among nations is as Christian an event as Christianity is a Jewish event - just because the Enlightenment grew out of Christianity doesn't mean that Christianity owns it. And Christianity does not own America.