Some of you are getting closer to what the Bible Belt is and was.
Consider the immediate aftermath of the War Between the States and the period of Reconstruction for a moment. To understand what life was like in the former Confederate States of America, think Germany after World War II - devastation of most of the infrastructure, occupation by hostile soldiers, all government functions taken over by the occupation forces, foreigners (Northerners) taking economic advantages and the difficulty that most people had in meeting the basic needs of shelter and food.
Religion provided comfort to people whose lives had been demolished. It was about the only thing left that couldn't be destroyed or stolen. The Third Great Awakening, which was a Protestant event predominant in the South, had a strong influence on social and political activism. The Bible Belt was being formed.
During the Spanish American War and there was a general sense of reunification between former enemies. Veterans from the opposing sides, who had been celebrating their valor independently for years, began to have joint reunions. Colors that had been seized during the war were returned to former enemies and the two populations in general came together as a result of facing a common enemy of the nation.
But the generations that followed as the veterans died off became increasingly resentful of a continued economic dominance by Northern businessmen and a perception of Northern cultural disdain for the South. The White South began to coalesce their dislike of things Northern by creating a romanticized version of all things Southern - and that meant the Confederacy and religion. The two were joined at the hip and a formal ritualized version of the two became important elements of White Southern culture.
There was a good side to all of the religious fervor and Lost Cause reminiscing, but it also came with a much bigger bad side like Jim Crow laws, and the resurgence of the Klan that was very violent and practiced religious oppression.
The South, had prior to this, been pretty tolerant and liberal in the area of religious freedom. That mostly meant tolerating Christians with different ideas (snake handlers for example), but it included Jews who had strong communities in places like Atlanta.
Religion became a very dominant feature of public life - prayers began all public functions including government functions from the start of the day for the state legislature and public schools.
Everybody pretty much at least confessed to being a Christian of the Protestant variety, which means that
sola scriptura was the name of the game.
By the time H. L. Mencken started covering the Scopes trial, which was at least partly a battle between Protestants of the Modernist sort and those of the Fundamentalist type, the "Read your Bible Every Day" kind of Christianity was thoroughly ingrained throughout the region. In fact, a side issue at Scopes' trial was the removal of a sign from the courthouse wall that said "Read Your Bible".
Mencken, that gleeful misanthrope who hated religion and thought all Southerners were "sub-human", coined or popularized the phrase "The Bible Belt" while he was writing about the Scopes Trial. It was obviously not meant as a compliment.
So, as originally used, the phrase meant that part of the former Confederacy (and some parts of some bordering states) in which the population was predominantly of one or the other Protestant faiths, and in which the presence of that religion's tenets were pervasive in public life.
In the historically correct meaning of the phrase "Bible Belt", all those good Mormons in Utah are not living in the Bible Belt, nor are the Roman Catholics in South Texas. They are not Christians of the sola scriptura brand, did not undergo the social and political changes brought on by the Third Great Awakening; and a geographic grouping of them does not constitute the Bible Belt any more than would a grouping of Muslims, Jews or Pastafarians.
The Scopes' trial was the height of the dispute between modernists and fundamentalists. There were major rifts within and between the major groupings of Protestantism, with the issue of evolution being one of the major tests between liberal and conservative, doctrinally appropriate and heretical. As a result of that dispute, and the verdict of the Scopes trial, the fundamentalist sola scriptura kind of religion became even more pervasive in the laws and culture in the South. Fundamentalists, and Southerners in general, were on the defensive and started pressing their legislatures for laws similar to the Burton Act.
In the late 1960s is when a lot of the quasi-official state religion of fundamentalist Christianity in the South was killed off by court rulings. It died a hard death, but it did die. That pervasive nature of a specific kind of religion in all things public, to include all things governmental especially, is gone.
The court rulings coincided (maybe even spurred?) the Fourth Great Awakening which again affected the Protestants. The Southern Baptist, the bastion of conservative Fundamentalist Christian churches, became politically powerful while the more liberal branches like Methodist and Episcopalians lost members and influence. At the same time, the SBC began growing outside of the South. As they grew nationally, so did the focus of their political and social activism.
Now, there are still plenty of religious people of the sola scriptura sort in the South. But they are also cropping up in many other parts of the country, so much that the Southern Baptists now call themselves the North American Baptists. And the Baptists question how much of the perceived (and proclaimed) religious fervor in the South is genuine adherence and how much is just a cultural lingering.
What are we to make of this? Well, a closer look at the question Gallup asked reveals the limits of the data. The people Gallup interviewed were asked: "Is religion an important part of your daily life?" There was no definition of "religion" offered, and certainly no test of doctrinal understanding or commitment. The responses from the Bible Belt surely include those generated by cultural Christianity. In the South, being "raised right" includes knowing how you are supposed to respond to a question like that posed by Gallup.