WHAT makes a person go to jail for his beliefs and fortifies that person to hold strong while the moments of life -- weddings, births, deaths -- pass by?

Essam el-Erian missed six and a half years of freedom for his political work. In that way he has something in common with those who accepted prison to call for freedom from the oppression of the Soviet Union. He shares something in common with Riad Seif, a former prisoner in Syria who was recently released after serving more than five years for his democracy work, and Ayman Nour, sentenced to five years hard labor in Egypt after challenging the governing party's monopoly on power.

But Mr. Erian's push for democracy does not sit as well with those in the West, namely the United States and Europe, because while he calls for free elections, freedom of speech and due process, he freely admits that he wants those political changes to help bring Islamic law to Egypt. His critics believe that religious-based organizations like his want to use the mechanics of democracy to achieve power, but have no intention of actually supporting democratic values or practices.

''It is a duty to do it, it is a religious duty,'' Mr. Erian, 52, said when asked about his motivation. ''I put my first footprint in this way more than 35 years ago, to save myself. Of course it may be difficult to understand, but secondly, it is to save our people. Our people look like slaves to dictatorships, and Islam came to release people from slavery to be only slaves of the God.''

Mr. Erian is a leading figure within the Muslim Brotherhood, a fundamentalist Islamic organization that is technically illegal in Egypt but is officially tolerated. He has worked for decades to help the group emerge from years in the underground as Egypt's only real political opposition. He was released in October after his latest stretch in prison, five and a half months, and got right back to work, rejoining the ranks of the Brotherhood just as its members won 88 seats in Parliament, a small step toward realizing his dreams.

Yet, facing an increasingly intolerant government, Mr. Erian operates on a short leash, which can snap back at any time. On Wednesday, for example, as he tried to board a flight to attend a conference in Bahrain, he was stopped at passport control and denied permission to leave the country. No reason was given.

If he has his way, the Brotherhood will follow in the footsteps of Hamas, the militant Palestinian organization that won parliamentary elections in January. And that, critics say, is precisely the problem. The Brotherhood, they say, is professing fealty to democracy only to gain power, after which it intends to dismantle democratic practices.

Mr. Erian is clear about his agenda: transforming Egypt into his vision of an Islamic state, where the Koran is not just the basis of all law, as in Egypt today, but where it is the law. But he rejects those who say he and the Brotherhood are antidemocratic.

''The main goal of Islam is to liberate people,'' he said during an interview in the office of the Muslim Brotherhood on the banks of the Nile here. ''The way that you can apply liberty in your life now is democracy. All are linked. They come from the same source. Of course you can find many, many Muslims, or the majority of Muslims, who do not understand that, but Hassan al-Bana, who is the founder of the Muslim Brotherhood, said that freedom, liberty is one of the main pieces of Islam.''

MR. ERIAN was born in a small village in Giza, called Nahya, not far from the Pyramids. His father was a teacher, and like most of the people in the village, he was raised observant, praying five times a day and fasting during the holy month of Ramadan. But he heard the call -- to faith and politics -- after an accident. He said he was about 15 years old when he was nearly electrocuted by touching the exposed wires on a lamp.

''I went to the street alone and asked myself, 'What am I living for?' ''

For a teenager, the answer was found in the Muslim Brotherhood. ''When I found the Muslim Brothers, of course, it was very good help for me because I put all my efforts in clear strategy, clear techniques, clear tasks,'' he said.

Mr. Erian, one of the two brightest students in his village, was selected to study medicine in Cairo. While in school, he joined a whole class of young people who were working to transform the Brotherhood with pragmatic ideas and a strategy of pushing the group into politics. Alarmed, the government struck back, and Mr. Erian, as the vanguard of that new generation, often felt the heat personally. He eventually earned a master's degree in clinical pathology but never managed to finish his medical training because he was sent to jail, he said.

Mr. Erian served in the Egyptian Parliament from 1987 to 1990 as an independent, because the Brotherhood could not run candidates as an outlawed group. In 1995 he was sentenced to five years in prison for belonging to a banned group that sought to create an Islamic state in Egypt. That conviction automatically disqualified him from running for public office again.

The Brotherhood has a violent past, and was outlawed after its members tried to assassinate Gamal Abdel Nasser even before he became president. Now it has to contend with that not-so-subtle undercurrent of suspicion that Mr. Erian and others are opportunists looking to ride the coattails of democracy to power -- with no intention of then ever letting go. American policy makers have gone so far as to say they refuse to speak with the Brotherhood because of its illegal status. Recently, however, a British publication, The New Statesman, published documents on its Web site leaked from the British Embassy in Cairo in which an analyst there said that it was time to begin at least an informal dialogue with the Brotherhood.

MR. ERIAN bridles at the policy of opposing the Brotherhood because of what it might do, saying it diverted attention from more substantive issues. ''The problem is that our rulers in this region are in power for half a century or more, supported by the states and they don't leave power,'' he said. ''You are afraid that somebody else can come to power and not give away. Let us put guarantees to prevent this. Don't prevent people from change because you are afraid or because you are weak. Strengthen yourself, don't weaken the others.''

Mr. Erian has picked supporters outside Islamic ranks. Saad Eddin Ibrahim, a Western-educated pioneer in pressing Egypt's government for accountability who found himself imprisoned some years ago for his trouble, said he trusted Mr. Erian. ''Probably because of his moderate attitude and his outstanding leadership qualities, Mubarak's regime put Essam el-Erian behind bars,'' wrote Mr. Ibrahim while he was in jail. ''So, let all the free people in Egypt and the world unite with him.

''Hands off Essam el-Erian, Mubarak!''

That call never did resonate, in Washington or Europe. But Mr. Erian said he did not care much, because he took the risks in support of his faith. ''It is only to obey my God,'' he said.

Photo: ESSAM EL-ERIAN (Photo by Shawn Baldwin for The New York Times)