More than 15,000 teen-agers from around the nation and abroad gathered at the D.C. Convention Center yesterday to celebrate their Christianity, while across town the hotel where many of them were lodged was shut down by a fire.

Youth Congress '85 proceeded with its all-day program of sermons, songs and speeches, apparently uninterrupted by the fire at the Washington Hilton where more than 3,500 of the youths were housed.

The teens, representing all 50 states and 12 foreign countries, came dressed in pastel shorts, blue jeans, casual shirts, sandals and tennis shoes. They sported cornrows and crew cuts and wore pop jewelry and crucifixes. Most arrived in church buses, some from as far away as Oregon. Each paid $179 for their hotel room, registration and some meals.

The five-day student leadership conference, which will end Tuesday, is sponsored by two large Christian youth groups: Youth for Christ and Campus Crusade for Christ.

"A lot of kids are here just to have fun," said Brom Nikrad, 17, of Minneapolis. "But people are finding out just by being here that you can learn a lot about the Lord."

Young people who came to the conference seeking spiritual knowledge had scores of workshops and seminars to choose from, on topics such as "Developing a Lifestyle for Changing the World" and "Overcoming Peer Pressure."

Volumes of books, cassettes, posters and magazines were on sale on the first floor of the convention center. And an array of entertainers and ministers addressed the youths in song and sermon.

There was a healthy selection for anyone who might have come to be entertained. After singing along with gospel singer Tramaine Hawkings and laughing along with humorist Pat Hurley in the convention center, many of the youths went off to see the city on tours sponsored by conference organizers.

"It's wonderful to be swept up in the experience of God," Bill Bright, president and founder of Campus Crusade for Christ, said to an audience of more than 12,000 young people. "But experience without obedience to God is empty.

"When you truly make Christ the Lord of your life," Bright said, "you cease doing certain things."

At least one young woman said the conference exposed her to "certain things" she had considered "un-Christianlike."

"I thought I was at a rock concert," said Carletta Oliver, 16, of Harwood, Md., referring to some of the upbeat rhythms played by some of the musical entertainers. Oliver said she was taken aback by the foot-stomping, hand-clapping response to some of the music.

"I've heard about these things, but I've never seen them." Oliver said.

A devoted fan of rock superstar Prince, 16-year-old Faith Weiser of Minneapolis, said she enjoyed the music yesterday, adding that Prince's "lyrics aren't good, I just like the beat."

Christian teen-agers are just ordinary kids, Weiser said.

"We accept people for who they are," she said. "But I worry a lot about being accepted myself. If people made fun of me as a Christian, I wouldn't know what to say."

Rich Miller, a spokesman for Campus Crusade for Christ, said the conference was giving the teen-agers "the tools to understand themselves and to help other people. We're teaching these kids to love."