The epiphany of Said Ferjani came after his poor childhood in a pious town in Tunisia, after a religious renaissance a generation ago awakened his intellect, after he plotted a coup and a torturer broke his back, and after he fled to Britain to join other Islamists seeking asylum on a passport he had borrowed from a friend.

Twenty-two years later, when Mr. Ferjani returned home, he understood the task at hand: building a democracy, led by Islamists, that would be a model for the Arab world.

“This is our test,” he said.

If the revolts that swept the Middle East a year ago were the coming of age of youths determined to imagine another future for the Arab world, the aftermath that has brought elections in Egypt and Tunisia and the prospect of decisive Islamist influence in Morocco, Libya and, perhaps, Syria is the moment of another, older generation.

No one knows how one of the most critical chapters in the history of the modern Arab world will end, as the region pivots from a movement against dictatorship toward a movement for something that is proving far more ambiguous. But the generation embodied by Mr. Ferjani, shaped by jail, exile and repression and bound by faith and alliances years in the making, will have the greatest say in determining what emerges.

Their ascent to the forefront of Arab politics charts the lingering intellectual and organizational prowess of the Muslim Brotherhood, a revivalist movement founded by an Egyptian schoolteacher in a Suez Canal town in 1928. But intellectual currents that once radiated from Egypt now just as often flow in the other direction, as scholars and activists in Morocco and Tunisia, perched on the Arab world’s periphery and often influenced by the West, export ideas that seek a synthesis of what the most radical Islamists, along with their many critics here and in the West, still deem irreconcilable: faith and democracy.

More often than not, they are asking societies for trust that, given the exper...