Why do Atheist seem to know more about the bible than Christian ? Have they something to prove ?

Some needed to prove to themselves first why they don't believe. Most are born within a religion (christianity in this case). Many atheists went to religious school, attended church as children etc. And they were paying attention. Sometimes curiosity made them question things that people who remained of faith did not need answers to. Atheists tend to read the Bible from many points of view. First as believers, then to answer questions, then to dispute the beliefs, then as literature and dissect it. Whereas Christians who go by the book are often seen as dogmatic, part of a cult or heretics (most Christians who belong in the big denominations don't read the whole Bible because they learn what they need to at their church). It's complicated dynamics. One example I can give you is my grandmother and myself. She's illiterate so can't really read the Bible, but she's a Christian. I went to school and had religious lessons since I was a kid, I can read the Bible and ended up being an atheist. So one could say that I know more about the Bible than my grandmother.