“To tell you the truth, I don’t think of him as Brotherhood, I think of him as Mohamed Beltagy,” Nour told me when I visited him at his apartment, a dim, cavernous place full of artsy bric-a-brac on a side street in Zamalek, an elegant enclave in central Cairo. In November 2010, Nour chose to boycott the last parliamentary elections held under Mubarak, knowing they would be fraudulent. But he came out to support one person’s campaign: Beltagy’s. On election night the two men were standing outside talking when “someone threw a rock the size of a football, and it went right between our noses,” Nour told me. “Thank God it missed. I am sure it was meant to kill one of us.”

When I visited Beltagy’s home, on an unpaved street in a newly built middle-class area in eastern Cairo, I was struck by the contrast with Nour’s Zamalek hideaway. After greeting me at the door, he led me into a reception room where the walls were painted an unusual two-tone pattern of red and gold. The room was large but elegantly spare, with little more in it than a cabinet full of his medical books. His wife, Sanaa Abdel Gawad, joined us, along with their 16-year-old daughter, Asmaa. Some of Beltagy’s Brotherhood friends had told me that he was “democratic” with his family, and indeed, his wife and daughter did not have the cowed, compliant look you sometimes see in women from conservative families.

At one point in my visit, Sanaa — a composed, wryly smiling woman in a white head scarf — took the floor to describe a terrifying night raid by the police last year, just after the Tahrir protests began. It was 2:30 a.m., she said, and she and her four children were awakened by loud banging and shouts of “Police!” She gathered the children and opened the door. A dozen armed officers streamed into the house, demanding to know where her husband was. “I told them I didn’t know,” Sanaa said. In fact, he had left hours earlier, having received word that Brotherhood leaders were being arrested all over Egypt, to prevent them from taking part in the protests. “They searched every room,” Sanaa told me. “They stayed two and a half hours and filled five bags with our papers and belongings.” Two officers stood by the door, complaining that they were tired. “Our youngest boy wanted to show these men he was not afraid,” she said. “So he asked them, ‘Which supermarket did you buy those guns at?’ ”

If the revolution gave Beltagy a national reputation, it also set him on a collision course with the Brotherhood’s old guard. On the night of Jan. 25, hours after the first protests began, several young Brotherhood members arrived at the group’s main office, hoping to find a senior official who would come with them to Tahrir Square. Only Beltagy was willing. As it turned out, he had already been protesting and was on his way back. A few hours later, he and his young friends were forced out of the square again, in a cloud of tear gas. “When I left the square, with thousands of young people, I was amazed to see that they did not go home,” he told me. “They just stayed in the street, chanting, ‘The people want the fall of the regime.’ That was when I knew I was in the presence of a new generation capable of writing history with its own blood.”

Two days later, after protesters fought their way back into Tahrir Square, Beltagy was with them. He stayed there, sleeping on a thin pallet in the winter mud alongside other protesters, until Feb. 11, when Mubarak stepped down. He was the only Brotherhood leader to do so. The group was notoriously slow to embrace the revolution, discouraging its members from going until much of Egypt was in open revolt. Beltagy became their de facto spokesman for the revolution, meeting regularly with the National Association for Change, a group formed by Mohamed ElBaradei, the Nobel laureate and avowed liberal. He even took his wife and children to the square — though they did not sleep there — and two of his sons were injured in the Battle of the Camels, the bloody fight with Mubarak supporters on Feb. 2.

After Mubarak was forced out, Beltagy gained a much broader following, both inside and outside the Brotherhood. He and his young revolutionaries seemed to represent the promise of a more open, more liberal-minded Islamist movement. In March, the new prime minister, Essam Sharaf, went to Tahrir Square to tell the crowds he would “draw his legitimacy” from them. In an image that became iconic in Egypt, he was photographed standing next to Beltagy.

Yet after a moment of euphoria, Egyptian politics began to polarize into secular and Islamist camps. The Brotherhood retreated from the protests, and many of its members banded together with hard-line Salafis to denounce their rivals as godless. Its leaders began sounding arrogant, triumphal notes. Sobhi Saleh, a popular Brotherhood figure from Alexandria, said in May that the group would dominate the next government and would implement Islamic law. He also said Brotherhood men should only marry Brotherhood women, to ensure that their children stayed within the fold. ElBaradei and other secular leaders began complaining that Egypt was on its way toward Islamist tyranny.