By MARC LACEY

When 800,000 of their countrymen were killed in massacres that began 10 years ago this week, many Rwandans lost faith not only in their government but in their religion as well. Today, in what is still a predominantly Catholic country, Islam is the fastest growing religion.

Roman Catholicism has been the dominant faith in Rwanda for more than a century. But many people, disgusted by the role that some priests and nuns played in the killing frenzy, have shunned organized religion altogether, and many more have turned to Islam.

''People died in my old church, and the pastor helped the killers,'' said Yakobo Djuma Nzeyimana, 21, who became a Muslim in 1996. ''I couldn't go back and pray there. I had to find something else.''

Wearing a black prayer cap, Mr. Nzeyimana was one of nearly 2,000 worshipers at the Masdjid Al Fat'h last Friday. The crowd was so large that some Muslims set their prayer mats on the dirt outside the mosque and prayed in the midday heat.

The Muslim community now boasts so many converts that it has had to embark on a crash campaign to build new mosques to accommodate all of the faithful. About 500 mosques are scattered throughout Rwanda, about double the number that existed a decade ago.

Although no accurate census has been done, Muslims leaders in Rwanda estimate that they have about a million followers, or about 15 percent of the population. That, too, would represent a doubling of their numbers in the past 10 years.

Muslim leaders credit the gains to their ability during the 1994 massacres to shield most Muslims, and many other Rwandans, from certain death. ''The Muslims handled themselves well in '94, and I wanted to be like them,'' said Alex Rutiririza, explaining why he converted to Islam last year.

With killing all around, he said, the safest place to be back then was in a Muslim neighborhood. Then as now, many of Rwanda's Muslims lived crowded together in the Biryogo neighborhood of Kigali.

During the mass killing of Tutsi, militias had the place surrounded, but Hutu Muslims did not cooperate with the Hutu killers. They said they felt far more connected through religion than through ethnicity, and Muslim Tutsi were spared.

''Nobody died in a mosque,'' said Ramadhani Rugema, executive secretary of the Muslim Association of Rwanda. ''No Muslim wanted any other Muslim to die. We stood up to the militias. And we helped many non-Muslims get away.''

Mr. Rugema, a Tutsi, said he owed his life to a Muslim stranger who hid him in his home when members of the Interahamwe militia were pursuing him.

Mr. Rugema said two imams had been arrested outside Kigali on charges of taking part in the massacre. But both were released within about two years for lack of evidence. ''We are proud of how Islam emerged from the genocide,'' he said.

For all the gains Islam has made, no one is suggesting that it is about to supplant Christianity as the country's leading religion. Catholicism, which arrived in the late 19th century with the White Fathers order of the Roman Catholic Church, remains deeply embedded in the culture.

On Palm Sunday, worshipers on their way home from Mass lined the roadways throughout Rwanda with fronds in their hands. They included people like Mediatrice Mukarutabana, who survived a massacre in her church that she says has made her even more observant now.

''God saved me,'' she said after the morning Mass at St. Francis Xavier Church in eastern Rwanda. ''He was testing my faith. Since the genocide I've been transformed. I can endure more now. I have more of a connection with God.''

Ms. Mukarutabana's church has a new pastor as well. The one who was there during 1994, a Spanish priest, tried to persuade the attacking militias to spare his congregation. He even offered them money if they would go away. But the militias would not relent.

After a standoff, the attackers offered the priest the opportunity to leave safely on his own, and he fled.

Ms. Mukarutabana said she had felt let down by the priest's decision to leave the congregation behind but understood his fear. ''We thank him because he made every effort to save us,'' she said. ''But when it came to the 11th hour, he blessed us and left us to die.''

Church leaders acknowledge that attendance at many parishes dropped after the killing rampage. They have taken pains since 1994 to teach a message of healing and to distance the church from clergy members who were implicated in the killings.

They say that after a period of decline, they are now slowly rebuilding attendance, and that Christianity continues to play an important role in the recovery from 1994.

The pain of 1994 lingers, though, and since the turmoil in Rwanda subsided in the late 1990's many, like Mr. Rutiririza, have been looking for an alternative to Christianity. But in a country where Christians account for about three-quarters of the population, Mr. Rutiririza found, as others have, that conversion can be a difficult and complex process.

He said he was ostracized by his Methodist congregation after he decided last year to become a Muslim. His wife remained with the Methodists, he said, while his children joined him at the mosque. Neighbors steer clear of him, he said, now that he has left Christianity.

The community that Mr. Rutiririza joined is a largely self-sufficient group, receiving relatively small amounts of aid from the larger Muslim world. Libya built a grand cultural center for Muslims in Rwanda more than 20 years ago, and Saudi Arabia provides financing for some of the mosques.

It is also largely an inward-looking group, and not a likely candidate for harboring cells of Al Qaeda. While Rwanda's Muslims say they follow the travails of Islamic adherents in other parts of the world -- the wars in Afghanistan and Iraq, for instance, and the conflict in the Middle East -- their primary focus is on their own struggle to put their lives back together.

''Our first priority is our country,'' Mr. Rugema said. ''Muslims in other countries face many problems, too, but we focus here more than on Afghanistan or Iraq. In Rwanda there is no Al Qaeda. We have too many problems to deal with. Mixing killing and religion -- we don't believe in this.''

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