What is a Fundamentalist, and why is being one such a problem?

Depending on which 'ism' to which one is referring, it could be a huge problem or a wonderful, right thing to be a "fundamentalist". For instance, a fundamentalist about mathematics is good. That means the person keeps to all of the fundamentals of math as we know it. Shifting from fundamentals and one ends up with catastrophes like New Math ;)

In religion, fundamentalism means that you take your holy book(s) seriously.

• If that holy book is all about loving God and loving people as you would want to be loved, it can't be bad ... only good ... to dive deep into the fundamentals. The issues arise when we don't apply all the fundamentals, but rather pick and choose the parts we want to apply/believe - the parts that fit our personal liking. 
• If that holy book is all about forcing all to follow that religion or die, and is about the degradation of women, and killing certain sinners, and suicide bombing, and establishing a one world order that is dominated by that religion ... fundamentalism is scary! We kind of want the latter holy book to be lightly followed, skipping the nasty stuff. 

But let's be clear - if followed only partially,

it's not really that religion at all.

Fundamentalism is often called "extremism" when it's really not. Fundamentalism is really just a following of the entire religion per its full direction.

So, one should want the former "loving" holy book to be followed fully - fundamentally. But one hopes the latter "dominating" holy book is not exercised fully.

By following all of it, the follower of the former becomes more loving, more selfless, and more servant-hearted, drawing others toward it because they want them to have the same love in their heart, and they want others (even enemies!) to be saved for eternity.

By following all of it, the follower of the latter strongly considers killing people from two other major religions until they are wiped from the face of the earth. They are a religion of peace only in that - once everyone is one of them or dead - things are peaceful. So "peace" is the goal.

In summary, "fundamentalism" is good if what the holy book says is true, having been fully inspired by a good, holy, loving (and just) God. "Fundamentalism" is bad when the holy book is built on lies and is no way a reflection of the real God - the one true God.

Just so we're clear.

Lord, draw them to you. And use me to share your truth and the free gift you offer.

In Jesus name. Amen.