In assessing what lies in the future for Pakistan and India, the role that the leading religion plays in each country—Islam in Pakistan, Hinduism in India—cannot be overlooked. In both nations, competing moderate and extremist religious views have been a wellspring of controversy and have fueled debates about what place religion should have in each nation’s political institutions.

Islam has been a fundamental part of Pakistan’s identity since before the nation’s founding in 1947. Pakistan was established as a country where Muslims, who constitute 97 percent of its population, could exist independently of Hindu-dominated India. Islam gained further official influence under General Mohammad Zia-ul-Haq, the military ruler of Pakistan from 1977 to 1988. Zia sought popular support and legitimacy by embracing Islam as the basis of Pakistan’s laws, installing devout...