Atheist Ads

Please help me understand how this works.  A group of atheists are organizing to place anti-God ads on buses in London.  How do you place an ad against Someone you don't believe exists?

A hilarious part of the linked article states that there is "preferential treatment given to religion in British society."  Wow.  Seeing that Europe is one of the greatest bastions of secular liberalism in the world today, I find that very hard to believe.