​To understand the numbers, it is helpful to make a distinction between nominal and practicing Catholics. Nominal Catholics are those who call themselves Catholic, but are not practicing or living the faith in any real sense. This number, is going up as our population continues to grow. And that growth is mainly from immigrants, the majority of whom are at least nominally Catholic. Thus the overall number of Catholics is growing. 
 
But to be a nominal Catholic is not necessarily to be a practicing Catholic. And, though here in America, the overall number of Catholics is growing, the number of practicing Catholics does seem to be declining overall. Sadly, only about 25% of Catholics go to Mass each Sunday (down from close to 80% in the 1950s). And an even smaller percentage agree with all the teachings of the faith and practice their faith daily to a significant degree.

​Non-Catholic Christians have usually been termed Protestants in the English language, because they were collectively "protesting" something in Catholic teaching or practice. But of course, Protestantism is a very diverse group of tens of thousands of different denominations which span the theological spectrum: liberal to conservative. 
 
Thirty years ago Protestant denominations were largely broken into two groups: the more socially conservative "Fundamentalists", and the more liberal, both socially and theologically "Mainline" denominations such as: Lutherans, Methodists, Presbyterians, Episcopalians, et al. 
 
The Fundamentalist tended to draw their numbers more from the more conservative Baptist congregations, and a wide variety of independent and nondenominational groups. 
 
What tended to divide Fundamentalist from Mainline Protestants was how the Scriptures were to be interpreted. Fundamentalist tended toward a more literalist adherence to the text that was more suspicious to applying historical context or other interpretive principles for understanding the text. The mainline denominations moved rather dramatically toward such interpretive keys, so much so that many of them have arguably moved beyond the text itself, and, as such, have no problem permitting things which biblical texts unambiguously forbid, e.g. homosexual acts, homosexual marriage, women clergy, etc. 
 
During the 1970s and 80s, there was a protracted campaign in the media and wider culture to discredit fundamentalism as rigid, pharisaical, and out of touch. Fair or not, the fundamentalists began to adopt the term "Evangelical" in response. While today's Evangelical Christians are not simply synonymous with the "Fundamentalists" of the past, the term "Fundamentalist" has largely been replaced by the term "Evangelical." 
 
Today, despite theological differences between Evangelicals and Catholics, there is a lot of common ground on the moral issues. This has led to greater and one of the most fruitful sources of converts to the Catholic Church from Evangelical Christian denominations today. 
 
They bring with them a great love for the Lord Jesus, and a great love for Scripture and seek a more stable, historical, theological and sacramental framework of the Catholic Church in which to live their faith. 
 
These closer relations have led to the description of "Evangelical Catholicism." This term expresses an emphasis on the Scriptural and Apostolic origins of our holy faith. At the heart of Evangelical Catholicism is the concern to avoid becoming too self-referential, to always remain centered on Christ.