I would concur that they are a 'threat' to secularism, in that they believe and will preach in opposition to it, but the OP stated "Democracy, Secularism, and the rule of law, and civil rights". Using "and" makes it inclusive, and if even one of the four listed is not true then the whole statement becomes untrue. But I digress.
As for civil rights, I would say that depends on what rights you are looking at. Free speech, self defense, the right to life... all of these are fully supported by Christianity (both in general and specifically Evangelical Protestants) as far as I have ever seen. Now some of the newer ideas of civil rights, like homosexual marriage, are of course contested.
I also wonder what problem you - or people in general - have with someone spreading what they believe their God says? In the case of Christianity, the core message is that we humans are sinful by nature, that as such we deserve death (we all do die, that seems to be a fact), and that Jesus came to earth and died in our place so that while we may still physically die we can be brought back to life and live forever with him. That isn't a message that harms anyone, and it has to be taken pretty far out of context to be used to cause injury or lead people do commit crimes. Sure, people *have* managed to mangle it and other teachings to cause problems... but the same can happen with the philosophy of Darwin (survival of the fittest -> kill those who aren't fit) or numerous other ideologies.
And *if* Christians are right, but they kept the good news to themselves, then you'd be pretty pissed to find that out after you died. So they are in a catch-22, so to speak: tell people what they believe, and be persecuted for it, or don't and disobey their God as well as potentially harm people by not warning them.