the arabs and persians , separated by language and culture , have been religious rivals since the death of mohammed , the founder of islam , in 632 a.d. the sunnis , who make up more than 90 percent of the world 's 850 million moslems , revere mohammed as second only to god and view abu bakr , the prophet 's father-in-law and the man who became islam 's first caliph , as his rightful successor .