First, fellas, this figure is only half of what the estimates are. That's his first mistake.
The second mistake is that Christianity is on the rise, ahead of Islam and continues to outdistance it. Fox News did a segment on the statistics on 1/3/2003. Islam is NOT the fastest growing religion. By 2025 Christianity will have reached a point where Islam will never be able to catch up.
One fact the author seems to miss is that oppressive regimes, including Islamic regimes, hold most of their "converts" by force. It is illegal to be anything except Islam, for example. Not so with Christianity and democracy. You are still free to be a pagan or communist or socialist here last time I checked. Remember the "Gates test?" Open the gates of the US and which way do people go? Into the country. Open the gates of an Islamic or communist country and which way to the people go? Out as fast as they can.
Our biggest worry at this moment is not any idealogical threat; it is offering our soverignty to the UN. The second biggest threat is internal decay.