"I go to school, I go to Ukrainian Catholic church and practice my religion. I can go free here without someone following me on the way to church . . ."

Walter Polovchak, the 14-year-old Ukrainian boy who refused to leave this country when his parents returned to the Soviet Union two years ago this month, made his first public appearance yesterday at a House subcommittee briefing.

Composed and unsmiling, he wore his longish light brown hair in a blow-dried ducktail and stood very straight when being introduced to various congressmen. He wore a navy blue suit with vest, steel-rimmed glasses and a Casio digital watch. Sitting next to him was his 19-year-old sister Natalie, in a cool-looking, short-sleeved aqua dress.

They talked about fear and the young people of the Soviet Union.

"They do believe in God, but they're very scared," Natalie said. "You've got to keep it secret. A lot of people are really faithful, but they're afraid."

What about war? "They got the feeling there's gonna be a war sooner or later," said Walter. "They feel that way."

The two--who have the same broad, triangular face--were flanked by their lawyers, who made sure reporters didn't get to them for more than a word or two. They are under court order forbidding them to talk to the press, but this session certainly assured that the press would get a good look at them. You could spot the room far down the corridor by the TV floodlight spilling out the open door.

Rep. Peter Peyser (D-N.Y.) explained that Natalie, now 19, came to this country on her own passport when the family immigrated, and is free to stay. But Walter, who was supposed to return to the Soviet Union with his parents, Michael and Anna Polovchak, and a younger brother, wanted to stay, too. He ran away from home in Chicago to seek asylum on the eve of his parents' departure. They left after seven months because of language difficulties and the resulting job problems.

Since then, the Immigration Service, the Justice Department and the State Department have gotten into the act, not to mention the American Civil Liberties Union, which is fighting for the parents' right to have custody of their child. The issue is whether parental rights should prevail despite the Soviet Union's record of harsh treatment of dissidents.

Last December, the Illinois Court of Appeals ruled that the state had no right to assume custody of Walter. His lawyers have appealed to the state Supreme Court.

Meanwhile, Walter has been granted permanent residency status by the Immigration Service and has been living in Chicago with court-appointed Ukrainian foster parents. His sister lives there with a cousin. And to quell his lawyers' fears that the boy might somehow be kidnaped by the Soviets, Attorney General William French Smith this year signed a departure control order blocking his exit. All international airlines have been notified, for good measure.

Speaking in a rapid, nervous slur of words, but with little accent, the youth answered questions about life in the Soviet Union. Sometimes he didn't understand a question, and his cocounsel, Julian Kulas, would put an arm around his shoulder and whisper a translation.

"If they find out you go to church, the school makes you clean the floors, paint some desks and all kinds of stuff," he said of the Soviet Union. His sister explained: "If you're in school, you have to belong to Comsomol the Communist youth organization . If you're not in the organization, it's hard to finish school and get into college and find a job. They teach you shouldn't believe in God, that religion is poison."

Because they were Catholics, she added, they were watched constantly, given no freedom of movement.

"We came here to Washington, D.C. from Chicago , we took the plane without any pass, any trouble . . . In the Soviet Union it is impossible to go anywhere without a pass or permission. The KGB follows you." Though the Catholic churches in the Soviet Union were closed at the end of World War II in favor of the state-approved Orthodox church, there are still some 5 million Catholics in the Ukraine alone, Kulas said. Walter's parents apparently are still Catholic, and it was his grandmother, who had raised him until he was 12, who instilled him with the faith. The grandmother died a year before the family moved to Chicago.

Peyser produced a taped interview with an unidentified former KGB major who predicted that if Walter were to return to the Soviet Union, his life "would become without any exaggeration really miserable. First thing, the KGB officers will try to brainwash and to force him to cooperate with them to make propaganda . . . on the atrocities of capitalism."

If he resisted, he would wind up in a mental institution, "first, to confine him, to lock him up, and second to sedate him to the extent that probably he could cooperate in the future," the former KGB officer said. Then, the tape indicated, there are the teen-age labor camps and, when he becomes an adult, the gulags.

Another Soviet dissident, Vyacheslav Repnikov, testified that Walter could expect to be left alone at first while still in the international spotlight, but that later the KGB would either force him to recant about America or send him to a mental institution. Repnikov, who spent over a year in "a Leningrad madhouse," met dissidents there as young as 15.

Walter seemed to agree. Asked what he thought about returning to the Soviet Union, he said, "Probably the first day they'd make a big publicity show that I came back, but later . . . I go off to jail. If I went back there right now, I'd be all my life in jail."

His sister had the last word: "A lot of American people don't realize how much freedom we have here, they take it for granted."

She has graduated from high school now. She wants to become a nurse.