People in Christian culture surround themselves almost exclusively with other like-minded people. They do have some acquaintances who are non-Christians but these are not close friends. These acquaintances are most often neighbors, co-workers, and other people who aren't easily avoided.
A person immersed in Christian culture feels some tension during every interaction with a non-Christian. This is because they feel they must represent Jesus and win that person for Christ. They feel they should overtly and literally present the gospel during almost every interaction and they feel a certain amount of personal responsibility for that person's salvation. It's an enormous amount of pressure.
This is not to say that Christians will not seek out conversation with a non-Christian. They will. But it is usually for the ultimate purpose of "witnessing" and avid pursuit of presenting the gospel to them in no uncertain terms. They feel that merely being that person's friend isn't quite enough. As a result the Christian culture person feels much more comfortable with fellow Bible-believing Christians. (Catholics don't count. Are you kidding?)
Christian culture in large part glosses over the fact that Jesus hung out with the grossest, most immoral people in his society and that the people considered to be the most holy people of his time, the Pharisees, looked down on this. Even still, Christian culture feels unsure about having friends who are agnostic, atheist, undecided, gay, strippers, drinkers, smokers, Mormons, Jehovah's Witnesses, Muslims, or maybe just kind of skanky. These people could also be saved for all we know but in the eyes of Christian culture their lifestyle trumps this possiblity. (Where is the fruit in their life? asks the Christian, unwitting to his own sin in judging that person.) Christian culture indeed knows that the Pharisees missed the point of the gospel of Christ, but Christian culture members generally are not able to entertain the possibility that they themselves could be modern-day Pharisees.*
*Disclaimer: anyone can have Phariseeical attitudes and thus be sinning, even/especially people with silly blogs.