It's a communal thing - Christians are drawn to places where there are more Christians, atheists to places that have more atheists. No one religion or lack thereof is really changing in numbers, they're just self-segregating. There is a massive Christian community not far from me, and is swelling in numbers.
Yet, if you read Revelations in the bible, it actually does state that the world in the end times would be against God, and those with faith would be dramatically outnumbered and be oppressed or forced to denounce their faith through the equivalent of what we would call "peer pressure". So the OP does actually ponder what is discussed in Revelations, which details not only the extinction of Christianity, but mankind as a whole.
The Carnival of Shadows - Folk Punk from Asbury Park, New Jersey
http://www.thecarnivalofshadows.com
