Friendship with the World Dilutes Us
Too many believers remain close friends with the ungodly and because of this they don't grow past infancy and even regress. I tend to think that if we are not growing we are regressing, we never stay completely stagnant. In order to keep ungodly friends we have to remain silent about many things, laugh at their foolishness and indulge in their sin, if only by our lack of response to it.
I take "come out from among them", very seriously. The only reason to be in the company of an unbeliever is to witness Christ, unless we work with them, and then we can let our light shine brightly, so that they will see Christ in us.