A look at the different religious perspectives and beliefs that distinguish these two broad groups of Christians in the United States and why membership in evangelical churches has been increasing while mainline denominations are, at best, just holding steady. Here are the views of John C Green, author of Religion and the Culture Wars; Richard Cizik of the National Association of Evangelicals; and Mark Noll, historian and professor at Wheaton College.

author of Religion and the Culture Wars

The easiest way to explain the differences between evangelicals and mainline Protestants is to start with evangelicals, because evangelicals have a clearer set of beliefs that distinguish them than mainline Protestants do.

The term evangelical comes from the word "evangel" which is a word form in Greek from the New Testament that refers to the good news of Jesus Christ -- that Jesus came to save humanity -- and evangelicals have a particular take on the good news. That makes them distinctive from other Christians. It could be summarized, I think, with four cardinal beliefs that evangelicals tend to hold, at least officially.

One belief is that the Bible is inerrant. It was without error in all of its claims about the nature of the world and the nature of God. A second belief is that the only way to salvation is through belief in Jesus Christ. A third belief, and one that is most well known, is the idea that individuals must accept salvation for themselves. They must become converted. Sometimes that's referred to as a born-again experience, sometimes a little different language. Then the fourth cardinal belief of evangelicals is the need to proselytize, or in their case, to spread the evangel, to evangelize.

Now different members of the evangelical community have slightly different takes on those four cardinal beliefs. But what distinguishes the evangelicals from other Protestants and other Christians is these four central beliefs that set them apart.

Mainline Protestants have a different perspective. They have a more modernist theology. So, for instance, they would read the Bible, not as the inerrant word of God, but as a historical document, which has God's word in it and a lot of very important truths, but that needs to be interpreted in every age by individuals of that time and that place.

Mainline Protestants tend to also believe that Jesus is the way to salvation. But many mainline Protestants would believe that perhaps there are other ways to salvation as well. People in other religious traditions, even outside of Christianity, may have access to God's grace and to salvation as well, on their own terms, and through their own means.

Mainline Protestants are much less concerned with personal conversion. Although they do talk about spiritual transformation, they'll often discuss a spiritual journey from one's youth to old age, leading on into eternity. So there is a sense of transformation, but there isn't that emphasis on conversion -- on that one moment or series of moments in which one's life is dramatically changed.

Finally, mainline Protestants are somewhat less concerned with proselytizing than evangelicals. Certainly proselytizing is something they believe in. They believe in sharing their beliefs with others, but not for the purposes of conversion necessarily. The idea of spreading the word in the mainline tradition is much broader than simply preaching the good news. It also involves economic development. It involves personal assistance, charity, a whole number of other activities.

But on many points, evangelicals and mainliners are sometimes hard to tell apart, because there are people in the evangelical tradition who are somewhat more modernist and tend towards the mainline. We often refer to them as liberal evangelicals. But then there are also people in the mainline churches who have a more traditional, or conservative perspective. They're sometimes referred to as evangelical mainline Protestants.

So this is a little bit confusing here, because the two communities are not as completely distinct as some might argue. But there are clear distinctions at the core of each tradition, which allows us to recognize them as different approaches to Protestantism.

Historian, Wheaton College

Why are mainline Protestant churches going along at a sort of steady pace and even declining, and evangelical churches are definitely seeing an increase?

The churches that are known as evangelical today are descended from the mainline Protestant churches of the 19th century. When a distinction is made between evangelical and mainline churches, it's not a hard and fast distinction. There are many, many evangelical mainline Protestants.

But the mainline churches are traditional. They are less entrepreneurial, less flexible in relationship to cultural [issues], and have, for reasons of belief and practice and organization, not fared nearly as well in the postwar world as have more self-consciously, self-identified evangelical churches.

National Association of Evangelicals.

… When you speak of mainline Protestants and what their beliefs are about the Bible, is it basically the difference between believing that the Bible is an interesting set of stories, versus a set of laws, a set of truth? …

That may be a layman's definition. But I think there's one way to understand the evangelical view of the Bible. It is viewed as the objective authoritative word of God, as opposed to the mainline Protestant view called neo-orthodoxy which holds, you see, that the Bible becomes the word of God in a kind of existential encounter with it.

So that's the distinction. It doesn't just become the word of God when you have an experience with God or an experience with the Word. It is objectively, authoritatively the word of God. That's what distinguishes evangelicals from, say, mainline Protestants. …

Let's talk about the old churches versus the new.

People have wondered, why are conservative churches growing? The answer is, they offer moral certitudes in a world without any certainties, it seems. They offer moral absolutes to people who are looking for moral guidance, and a way to live in a crazy, mixed up world. Then they combine it with contemporary music and worship. It's appealing, and they're growing. There's a mega-church formed every two weeks in America. Meanwhile, mainline Protestantism, sometimes called the sidelines, is dying.