First, Arab isn't synonymous with Muslim. Islam did arise out of the Arab bedouin society and culture of 8th century Arabia, and the Koran was written in and studied in Arabic, so the influence of Medieval Arab culture and society is important, just as nomadic Caananite culture and society of .a thousand years earlier is important in shaping all the monotheistic religions. But to equate Arab (which is a cultural, social, ethnic and linguist identification) with Muslim (which is a religious identification) is like using calling Christians Jews, because Jesus and most of his earliest followers were Jews.
Beyond the historical aspects of the different identifications, there are many non-Muslim Arabs, and far, far more non-Arab Muslims than there are Arabs of all faiths combined. There are approximately one billion Muslims, but only about 200 million of those Muslims are Arabs. In fact, of the countries with the largest Muslim populations, only Egypt is an Arab country (in millions, %=percentage of national population that's Muslim; figures calculated from country profiles in CIA Factbook:
1.Indonesia (212m, 88%)
2.India (140m, 13%)
3.China (133m, 1%)*
4.Pakistan (157m, 97%)
5.Bangladesh (119m, 83%)
6.Egypt (74m, 94%)
7.Turkey (69m, 99%)
8.Iran (66m, 98%)
9.Nigeria (64m, 50%)
10.Ethiopia (32m, 45%)