This is a meaningless study. Why choose “millionaire” as the basis of the study? Two muslim oil billionaires have more wealth than 10 Christians worth two million apiece. Although all 12 are “millionaires” and the vast majority are Christians, the Muslims in this sample would still have more wealth. So the study is meaningless.
Muslims run a society that keeps the wealth more isolated than does Christianity and Judaism. Generally speaking.
In 2014 Christian population is 2.01 billion and Muslims are 2.08 billion.
http://www.religiouspopulation.com/World/