THE kerchiefed worshipers in St. Theresa's Roman Catholic Church intone a solemn hymn, and the evensong drifts outside to Gorky Street. There it trails along behind three Soviet soldiers out for an evening stroll. The waning sunlight glints off the red stars on their caps. It also casts a glow on the golden crown atop a steeple farther down the avenue.
The music fades as the soldiers near the ornate building. It is -- or, rather, was -- St. Casimir's Church, dedicated to the patron saint of this small, predominantly Roman Catholic republic on the Baltic Sea. It is now a museum of atheism, established and run by the Soviet state. Inside, a woodcarving depicts a monk in prayer, blissfully unaware of the suffering masses around him. A stained-glass window quotes a Lithuanian philosopher saying, ``There is no God.''
The museum's presence in the center of this centuries-old city is a constant reminder of the mark the Soviets have made on Lithuania and its main church. It has been 45 years since Soviet troops occupied the nation and imposed a communist government, ending Lithuania's two decades of independence.
It has been 30 years since the consecration of the bishop of Vilnius, Julijonas Steponavicius. But Soviet authorities never allowed him to take office, and he has been in exile in the small Lithuanian town of Zhagare for most of that period for asserting that ecclesiastical law -- not Soviet law -- should govern church affairs.
``He was sent to a remote area. He is still there, because he refuses to recognize the state laws,'' says Edvardas Juozenas, deputy director of the Lithuanian Council for Religious Affairs, the government's overseer and regulator of church affairs.
Government authorities are, of course, spotlighting the 45th anniversary of Soviet rule here as the ``liberation'' of Lithuania from ``bourgeois'' forces. But many Roman Catholics view the occasion as another tumultuous chapter in the troubled history of their country.
``Our situation is very difficult,'' said one worshiper in the Lithuanian capital.
Lithuania is wedged between Russia and Poland, which is predominantly Roman Catholic. Soviet authorities have seen to it that Polish unrest has not spilled over to Lithuania. But here, as in Poland, the Catholic Church has become a sanctuary for Lithuania's cultural heritage and, in some respects, its national identity.
For that reason, the government keeps a weather eye on church activities and waits for the extinguishment of religion that Marxism-Leninism holds is inevitable. Meanwhile, it restricts church activities and forbids religious instruction for the young.
``If you speak of the reality, even in the countries where Marxism-Leninism is not the ideology, there is a threat that the church will pass away in time,'' says Mr. Juozenas. But, he says, ``If someone says the church is oppressed here, it's not true.'' It is merely that ``certain historical processes are going on,'' he says.
Undeniably, the Catholic Church here has changed in the 45 years Lithuania has been a Soviet republic. Church property has been confiscated, chapels and monasteries closed, and some priests arrested. At present authorities permit only one seminary to operate and they limit numbers of applicants.
The number of priests declines yearly. Church officials say there are not enough to serve the country's 630 parishes.
The government also controls all printing presses -- and church publications. The Bible is issued infrequently in Lithuanian translation, and in limited numbers. The most recent press run was in 1972, when 12,000 copies were produced.
In 1982, Juozenas was quoted as saying a new edition would be issued ``soon.'' Now, he says, printing will be possible in 1987. He will not say how many copies will be produced.
Church leaders are awaiting the decision hopefully, because 1987 marks the 6th century of Christianity here.
Indeed, the government does make concessions to the church from time to time. For example, it occasionally allows church officials to travel to the Vatican and to Catholic gatherings in other countries.
A delegation returned from the Vatican only last month, says Juozenas. And the government is now printing 160,000 copies of Catholic prayer books, he adds.
The Catholic Church's problems stem from its own falling popularity here, as elsewhere, he says. And the Vatican, he claims, is trying to shift the blame for the decline to the Soviet government.
Church officials were unavailable for comment. One, on finding two American reporters at his door, said, ``Please. I won't answer questions. You can look at the church. See all there is to see. But no questions.''
A far different picture emerges from an underground publication called the Chronicle of the Catholic Church in Lithuania. Over the last 13 years, in more than 60 issues, it has described in detail a church in trouble. It claims the Soviet secret police have disrupted church festivals, infiltrated the seminary, and harassed and imprisoned believers.
Soviet authorities are unremittingly hostile to anyone suspected of involvement with the Chronicle. Recently, a 79-year-old Catholic was sentenced to four years in labor camps and two years in exile.
Although the exact charges against Vladas Lapienis are not known, they are believed to be based on his handwritten account of five years spent earlier in the labor camps for distributing the Chronicle.
Nevertheless, Juozenas plays down the importance of the publication.
``As far as the Chronicle is concerned, we don't see it here,'' he says. ``We don't know about it. It's difficult to say who is publishing this today. I only hear about it on Radio Vatican.''
Vatican Radio is also a sensitive subject here. Juozenas says it broadcasts anticommunist transmissions and is a forum for ``reactionary 'emigr'es.'' He professes to know nothing about jamming of the broadcasts by the Soviet government.
``I sometimes hear Radio Vatican,'' he says. ``I listen to those programs regularly. I have had no problems.'' He says ``atmospheric problems'' might cause reception problems from time to time. Nevertheless, three massive jamming towers jut up above the Vilnius skyline.
The Chronicle says the church has been ``desecrated'' by being turned into a museum of atheism. Church leaders have repeatedly asked that it be turned back over to them. Soviet authorities continue to reject the requests.
But there does seem to be a degree of official embarrassment, however slight, at the church's fate. A guide with the official travel agency Intourist pointedly does not mention the museum of atheism as she tours the area with a group of Americans. Instead, giving a sweeping gesture across the square on which the former church stands, she says that under Soviet rule, ``most of the buildings have been restored to their original purpose'' since World War II.