This is trendy scholarship but in no way the real story of the rise of Christianity. It is fringe scholarship intensely characterized by a thoroughgoing antisupernaturlaism. What is presented as sound scholarship is sadly error-filled, distorted and thoroughly biased. Furthermore, annually tens of thousands of illegal immigrants enter who are believers in Islam. In addition, Muslims tend to have more children than non-Muslims because most do not practice any form of birth control.
Conversion also adds to Muslim growth.
Most of these come from Catholic, Protestant or Jewish backgrounds. There are over 12,000 mosques in the US with at least one in each state. Many of these projects are financed by Saudi Arabia and Kuwait. In 1991 the US Armed Forces Chaplains Board recognized he American Muslim Council as an Ecclesiastical Endorsing Agency.