There were great advantages and yet great disadvantages to "Christendom." The advantage was that there was a common language for public moral discourse with which society could discuss what was "the good." The disadvantage was that Christian morality without gospel-changed hearts often led to cruelty and hypocrisy. Think of how the small town in "Christendom" treated the unwed mother or the gay person.
"The Missional Church," by conservative Presbyterian (PCA) pastor Tim Keller
(Hat tip: The Jolly Blogger)