I'm an atheist, but worked for years in an area with lots of evangelical Christians. You've got to remember that if they've been brought up in that culture, then many of the significant adults in their lives, starting with their parents, will have been preaching this as totally normal since birth.
I worked with plenty of very, very intelligent people who would argue about things like evolution, and despite having grown up a Catholic, I was surprised about just how much it's all intertwined with day to day life, and how important spreading the word is (I suppose the clue is in the title). At one point, I think I had at least a dozen people praying for my soul :)
It's such a huge part of life, and I'd argue a little bit cultish, so I'd be wary of judging anyone that is relatively young for still being in the thrall of that brand of religion.