As the fiery orange sun sinks behind the mountains, the stones of the 800-year-old Alhambra take on a rosy glow. Against the backdrop of the snowcapped peaks of the Sierra Nevada, the fortress' rugged towers stand out in the gathering dusk.

As the lights of this long-ago capital of al-Andalus--Islamic Spain--blink to life, about 30 men kneel in neat rows inside a whitewashed mosque atop a hill facing the Alhambra. Palms held upward, they recite the evening prayers and bend forward until their heads touch the floor. Behind a thin screen, the shadowy outlines of the women of the mosque move in the same time-honored rhythms.

These two hilltop edifices represent the past and present faces of Islam.

The Alhambra fortress, which the Moorish rulers of southern Spain began to construct in 1238, recalls the splendor and achievements of the golden age of Islam, when the youngest of the three great monotheistic religions held sway from the Straits of Gibraltar in the west to the banks of the Indus River in the east.

Across the ravine, the humble mosque, whose plain white walls and red tile roof make it virtually indistinguishable from its neighbors, testifies to the renewed vigor of Islam, a fast-growing religion with a worldwide membership of about 1.2 billion, including 2 million to 4 million in America, although some Muslim groups put the figure at 7 million.

It is the first new mosque in Granada in more than 500 years, yet its opening in July came at a time of profound questioning about the meaning and direction of Islam. The Koran, Islam's holy book, preaches peace and charity, but to some Western ears, the loudest voices in the Muslim world extol hate and violence.

Islam is hardly unique as a religion that has been twisted to justify killing. Historical circumstances help explain how terrorists have commandeered Islam as their cause.

Unlike some religions, Islam does not have a clearly defined hierarchy to pronounce authoritatively on matters of doctrine and interpretation. Backed by radical imams, Islamic militants can claim that their interpretation of Islamic teachings is at least as valid as others. Also, Islam recognizes no separation between church and state. That makes it easy for terrorists to cloak their political causes--over Palestine or the presence of U.S. troops in Saudi Arabia--in religious rhetoric. And the concept of jihad, which signifies much more than holy war, provides another convenient cover for killers and suicide bombers.

But scholars who have studied Islam and terrorism say none of these factors would matter if the Islamic world were not still suffering from a centuries-old crisis of confidence, born of the loss of its place at the forefront of world civilization.

"A conquering civilization doesn't have terrorism," said Seyyed Hossein Nasr, professor of Islamic studies at George Washington University. "The conquered have terrorism. If you look on the map, the question of Palestine, of Kashmir, of the southern Philippines, of Chechnya, they are all places where Muslims are trying to protect their land or their culture, and they are losing. They only have recourse to these horrible means."

Islam's current sense of itself as an embattled faith stands in stark contrast to its past. For at least 500 years, the Islamic world was the driving force in human development. While Europe struggled to emerge from the chaos left by the collapse of the Roman Empire, Muslim scientists, engineers and architects were the most advanced in the world, and Muslim rulers nurtured a cosmopolitan culture that was often remarkably tolerant of the Jews and Christians who lived in its midst.

The youngest of the three closely related faiths that believe in one god, Islam was born in the unpromising desert landscape of the Arabian Peninsula.

In 610, a 40-year-old businessman was on a monthlong spiritual retreat on Mt. Hira, near the Arabian city of Mecca, when the archangel Gabriel appeared to him and uttered the command, "Iqra"--"Recite."

God's words began to pour from the man's mouth.

Over the next 22 years, Muhammad ibn Abdullah received many such revelations, which his followers later wrote down, forming the Koran.

Christian and Jewish readers of the Koran are struck by the familiar names they encounter. Adam, Eve, Abraham, Moses, David and Jesus all figure into Muhammad's revelations, sometimes in ways that coincide with the Bible and sometimes in ways that are strikingly different.

"The three [religions] came from essentially the same place," said Frank Peters, a professor of Middle Eastern studies and religion at New York University. "They all worship the same God, as they all admit."

But to Muslims, Jesus is a prophet, not the son of God. And, although they accept the Jewish Torah and the Christian Gospels as inspired texts, they believe that these writings became corrupted with errors and falsifications. They see the Koran as the true word of God, delivered to Muhammad, who is God's rasul, or messenger. "This is the Scripture whereof there is no doubt," God declares at the beginning of the Koran.

Expansion, fissures