theocracy

In the Middle Ages, the Muslim empires stretching around much of the Mediterranean were theocracies, and the pope ruled most of modern-day Italy. But theocracies are rare today. Modern Iran and Saudi Arabia (and perhaps half a dozen others) are usually regarded as theocratic governments, since, even though Iran's president is elected by popular vote and Saudi Arabia is ruled by a royal family, the countries' laws are religious laws. But when a government tries to follow all the teachings of a single religion, things usually don't work out terribly well, so U.S. Constitution and Bill of Rights forbid using religion as the principal basis for democracy.