Skepticism is about doubting something, like religion. Skeptics doubt many things including creationism, homeopathy, astrology, and more. Doubting and exercising critical thinking is an important part of being rational. Doubt is totally opposed to unquestioning childlike faith that religious people value. Frankly, religions including Christianity and especially Christian Fundamentalism are just really unreasonable in very many different respects. That's why once people stop being childlike and start using adult thinking skills faith too often evaporates.