Just who are my friends? Are they Christians, or not? Or, are they some of each? If my friends are mostly Christians I am in danger of disappearing into a little, or not so little, Christian ghetto which will not be good for me at all.
We have only to read the Matthew, Mark, Luke and John to know that Jesus' circle of friends was by no means made up from the 'middle class', or 'in' crowd' of his time. His circle of followers were the disenfranchised, the scorned and outcast, people in need. After all, He shares with us, in no uncertain words, only those who are sick need a physician's healing power.
Our contact with non-Christians should not remain superficial. Friendship, when it happens, comes from a mutual liking and respect, well beyond having nothing more than common interests. It means being glad to spend time together, it means developing a sincere and deep trust in one another.
Friendship requires honesty. Friends do not need you to be 'up' all the time, instead they enable you to be more vulnerable.