In reality, none of today's religious groups go back to the original meanings of the Bible, even those who have that intent. Why not?
1) The original manuscripts do not exist and even if they did, it's impossible to translate complicated ideas literally from one language to another. Even simple ideas cannot always be translated literally between closely related languages like Germand and English. And religious ideas are rarely simple.
2) Our world view is so different than that of the people who wrote the Bible thousands of years ago, that we cannot even begin to understand what they "really meant" when they wrote. In addition, the categories of "fiction" and "nonfiction" did not exist at the time. Mythological stories were not intended to represent historical accuracy. That does not mean they were just "made up" as today's novels are. They were meant to elucidate on the humand condition. In a way, they were an early form of psychology. The purpose of such writings was to give meaning to life through metaphor and analogy.
That said, fundamentalists do intend to try, at least, to return to early versions of their religion. Unfortunately, they most often try to do that via a misinformed idea that taking their scripture literally brings them closer to the original meanings. This is far from the truth.
Regarding the difference between extremists and fundamenalists, I think that's an important distinction to make. It is the reason we have the words Islamist and Muslim, which carry hugely different connotations. I have noticed that many writers have started calling extremist Christians "Christianists" to differentiate them from other people who follow versions of Christianity.