Although both “mainline” and “fundamentalist” Protestant churches have many beliefs in common, they are different in very important ways. In general, the differences are such that mainline churches are less traditional and more “big-tent.” Let us examine a few of the ways in which this is so.
First, fundamentalist churches are much more convinced that Christianity is the only true faith. Of course, mainline churches believe that Christ is the savior of the world, but they are less dogmatic and exclusive about this. They believe that there may be other ways to salvation than through Christ. They are likely to think, for example, that a great man like the Mahatma Gandhi can be saved even though he was not Christian. Fundamentalists reject this idea. To them, salvation is only possible through Christ.
Second, fundamentalist churches have much more of an emphasis on being “born again.” This is important enough that fundamentalists are sometimes called “born again Christians.” The mainline churches are more likely to say that a person grows (or can grow) spiritually at a gradual rate throughout their lifetimes. To them, there is not usually one moment that is more important than the next in this process. To fundamentalists, by contrast, there is typically an identifiable moment in which the person accepts Jesus and is born again. This is in contrast to the gradualism of the mainline view.
Finally, let us look at the most important difference, the difference in views of the Bible. Fundamentalists believe strongly that the Bible is the literal word of God and that it cannot be wrong in any way. They do not accept the idea that parts of the Bible are meant to be understood as metaphors. They do not accept, for example, that the Bible might have resulted in part from attempts by people who had little scientific knowledge to understand how the world worked. This is why fundamentalists insist on things like the idea that the universe was created in 6 days and that humans and other species were created directly by God. By contrast, mainline churches are much more open to more liberal readings of the Bible. They are willing to accept that each word of the Bible may not be the literal truth.
In these ways, among others, mainline Protestant churches are much less traditional than fundamentalist sects.