Here's one possible reason:
The greatest single cause of atheism in the world today is Christians, who acknowledge Jesus with their lips and walk out the door, and deny Him by their lifestyle. That is what an unbelieving world simply finds unbelievable.
If our lifestyles can diminish the credibility of our witness, how can we present a credible witness?
Brenda Salter-McNeil addressed this question on the last night of Urbana. Unfortunately, I missed it, sick in bed with a high fever. But, the video is available here.
According to Brenda, if you are going to present a credible witness to the gospel of Jesus...
...we must name our fears
...we must start where we are
...we must move to a new neighborhood
...we must go out in God's power
I'll expand on these more later, but in the meantime, ask yourself, are we presenting a credible witness?