It struck me as I was reading that what seems to occur in this book is not that people change their ideologies but rather that other things (a romance, a friendship, money, power, success) become more important to them than their ideology. So although on paper they would still have the same ideology the determining factor in their life is no longer primarily their ideological belief but rather something else. This process also seems to be at the heart of the film 'The lives of others'.
Now it seems to me that although ideological shifts do occur this process goes on a lot in churches. So whilst some people who once believed the Christian message of Jesus come to a point where they no longer believe that, many more don't reject it but rather find that through their life other things become more important than their Christianity to them. So they pursue a relationship which they find becomes more important to them and then they get a bit of money and find that the acquisition of money becomes more important to them and then they find a cause and find that this becomes more important to them than their faith. What then occurs is that they actually are no longer living according to their ideology despite the fact that they would still say that they hold to that ideology.
I think as Christians we must be careful that we allow our ideology to dictate our actions and values rather than drifting through life and finding that it is our actions and circumstances which have determined our ideology.