Merriam-Webster define Unchristian as:
:not of the Christian faith
"What are Christians known for? Outsiders think our moralizing, our condemnations, and our attempts to draw boundaries around everything. Even if these standards are accurate and biblical, they seem to be all we have to offer. And our lives are a poor advertisement for the standards. We have set the gameboard to register lifestyle points; then we are surprised to be trapped by our mistakes. The truth is we have invited the hypocrite image." - David Kinnaman.
Unchristian challenges us to live lives that reflect Christ, to humble ourselves, and consider how we live our lives can be a road block to others developing a relationship with the a loving God.
"When outsiders claim that we are unchristian, it is a reflection of this jumbled (and predominately negative) set of perceptions. When they see Christians not acting like Jesus, they quickly conclude that the group deserves an unchristian label. Like a corrupted computer file or a bad photocopy, Christianity, they say, is no longer in pure form, and so they reject it. One quarter of outsiders say therefore most perception of Christianity is that the faith has changed for the worse. It has gotten off-track and is not what Christ intended. Modern-day Christianity no longer seems Christian." - David Kinnaman
UnChristian challenges each of and the church to carefully examine how we are representing Christ in our everyday lives. It is disturbing, challenging, and thought provoking. I highly recommend it for both individual reading as well as group study.