One common challenge to the idea of Christianity in general is the idea that many people are only Christian because they were born in a predominantly Christian society. This argument takes many forms. One common claim is that if the same person (who claims to be Christian) were born in Iraq, they would be a Muslim.
Other times, the claim might be more along the lines of the idea that someone is a Christian because their parents raised them that way. In other cases, they may attribute someone's faith to a reaction of some tragedy or trauma in their life.