One should first recognize that the early history of the Northern United States was very religious. The Puritans arriving along the Massachusetts coast in 1620, the Salem Witch Trials in Massachusetts during the late 1600's, the major Ivy League Universities, such as Harvard, Yale and Princeton, which began as Divinity Schools-(or Theology Schools), the establishment of the Dutch Reformed Church in New Amsterdam-(Dutch New York City in the 1600's), as well as their founding-(or co-founding) of New Jersey's Rutgers University in 1766. The Northeastern United States has a deeply rooted religious history that should not be overlooked or trivialized.
However, what distinguishes Northeastern Christian religiosity from Southern Christian religiosity-(i.e. The Bible Belt region, from Virginia, to Texas), may have to do with the economic and socio-demographic evolution of both regions.
The Northeastern United States, from its earliest beginnings, was-(and remains), the most urban region in America. It had the largest cities, great wealth generated from the earliest years of the Industrial Revolution, as well as helped to establish the workings of Modern Capitalism through trade, banking and finance. However, the older Northeastern Protestant communities, were becoming an increasing minority presence in this region of the country, when Roman Catholics from various parts of Europe arrived in huge percentages throughout the 1800's and early 1900's. Throughout the 20th century, the urban Northeast continued to attract many other Christian groups from Europe and the Middle East, as well as throughout other parts of the United States. Other religious groups, such as Jews from Eastern Europe, Muslims and Hindus, became a major part of the Northeast's religious demography. In other words, the religious history of the Northeastern United States became home to a massively heterodox and heterogeneous region-(What we typically call, "Multiculturalism"), which exists to this day.
The Southern United States had a different historical development, especially towards religion. The Southern U.S.-(from Virginia, to Texas, Florida, being the major historic and contemporary exception), has primarily been, a religiously and to some extent, mostly homogeneous region of the United States. The major ethno-religious groups living in the Southern United States before 1900, were centuries old Anglo-Saxon ethnic groups, such as the English, Scots, Irish (Protestants), as well as Scotch-Irish-(one could perhaps include Germans and the Dutch, to a lesser extent). The religion in the South was primarily Christianity, specifically, a small number of Protestant denominations, such as Baptists, Methodists, Presbyterians and Episcopalians-(to some extent). There of course were French Catholics in Louisiana, the historic presence of Spanish Catholics in San Antonio, Texas, as well parts of Florida. African-Americans became Christians, namely Baptists, as well as Methodists, during and after slavery.
The South has always been a more rural and pastoral region when compared with the Northeast; Industrialization, Corporate Capitalism and Urbanization, came to the South much later in its history. Although there were immigrant groups who migrated to the South during the early 1900's, such as European Catholics, as well as Jews from Eastern Europe, the religious and ethnic demographic composition was primarily Anglo-Saxon and Protestant and a sizable African-American Protestant population as well.
The Northeast, during the Industrial Age and beyond, has been rather reserved or less publicly expressive with regard to its religious identity. As a Northerner myself, I can say, from direct experience and observation, that our region of the U.S. is, generally speaking, less preoccupied with religion and religiosity and more concerned about mundane matters. The Northeast generally maintains a more progressive cultural existence which acknowledges the presence and indispensability of religion, though keeping the institution of religion away and distant from the public square. The Northeast (generally speaking) tends to view public expressions of religiosity as inappropriate and is usually more introspectively religious-(if religious at all).
The South, (perhaps due to its traditionally pastoral and mostly homogeneous ethno-religious composition), is very publicly expressive in vocalizing in their religious self-identity, namely their very conservative Protestant religious self-identity. Religion, (specifically Evangelical Christianity), in the South, is a near ubiquitous presence throughout much of its public life and plays a central role in the collective social and moral identity within many towns and even cities-(to some extent).
Today, the Southern United States has major cities and metropolitan areas, such as Charlotte, Atlanta and Dallas. There are sizable percentages of Roman Catholics and other religious groups living in the more urban and suburban areas of the South. However, the deep rooted homogeneity of the South, especially regarding a particularly more conservative form of Christianity.......is still a major and undeniable presence in this part of the United States.