He has focused in particular on the fundamentalist phenomenon, showing that since the 1970s fundamentalism has been a crucial force throughout the world and across religions—among Protestants, Catholics, and Jews as well as Muslims.
Han har särskilt belyst fenomenet fundamentalism och påvisat att denna sedan 70-talet utgjort en avgörande kraft världen över i skilda religioner som protestantism, katolicism, judendom och islam.