Refers to a follower of Islam. The term イスラム教徒 is more common in modern Japanese.
See also: イスラム教徒
He is a Muslim and never misses his daily prayers.
Compound of 回教 (Islam, literally 'Hui religion') and 徒 (follower). The term 回教 is an older name for Islam, derived from the Hui people in China.