The biggest reason people turn away from Christianity is the ugly self-rightousness, hypocricy, dupliciousness, and visable hate of people such as Jim Phelps and Tedd Haggard.  Wikipedia has a list of “fallen” prominent evangelicals from 1920 to the present. There were 35 people on this list: 

http://en.wikipedia.org/wiki/List_of_scandals_involving_evangelical_Christians 

They, along with pedophile Catholic priests, and Jerry Falwell and Pat Robertson claiming that immorality and anti-Christian groups were to blame for 9/11, do more damage to Christianity than all the lectures, debates and book tours of the militant athiests combined.

But then, unexpectantly, you come across the actions of Christians such as Shane Claiborne of the Red Letter Christians; or the Amish who forgave the killer who slew five of their little girls; or the Christian couple from Northern California, who, after their young son was gunned down and killed during a tour of Italy, donated his organs to the Italian people.  You hear about these things and you think, maybe they do know something I don’t know.  

If more Christians demonstrated the grace and magninimity of these folks, Christianity would have far more adherents.