One of my favorite Christian authors is J. Warner Wallace (Cold Case Christianity), who gave a fantastic lecture here at NC State in December. I was browsing his blog the other day and came across this piece where he describes an intriguing exchange between some skeptical readers of his blog.
In essence, one of the commenters came out and said in very plain words what the consequences are of an atheistic worldview. The bottom line is there is no foundation for morality. I recommend it as a quick read.