Christianity should really be all about Jesus Christ at the very heart of it. However, for most people today it is more regarding the '...ianity' bit at the end. Therefore, it has sadly turned into vanity, an extreme belief in one's own abilities or attractiveness to others. The Bible calls it 'pride', the opinion that one does not need help in life, least of all from God.
Atheism is purposly included in that list since those who are attracted to that faith believe that there is no God. Followers of those ways, seekers after Truth and Christians will find a wealth of material to help them grow in their current understanding.