SAO PAULO, BRAZIL -- The leading proponents of liberation theology in Brazil are taking their message to the Soviet Union as part of a plan to strengthen links between Socialist states and Latin America's progressive Catholic Church.

Franciscan friar Leonardo Boff -- who in March 1985 was condemned by the Vatican to a year of "obedient silence" for his criticism of the Catholic establishment and possible heresies in his published works on liberation theology -- leads the expedition that is spending two weeks in the U.S.S.R., visiting Soviet republics with the greatest proportion of Christians.

He is accompanied by theologian Brother Betto -- Carlos Alberto Libanio Christo -- who in 1985 published "Fidel and Religion," a detailed interview with Cuban leader Fidel Castro that has become a best-seller in Latin America and that he says has received special praise from Pope John Paul II. The book explores the possibility of alliance between South America's Christian poor and the Communist Party.

Brother Betto said the group of theologians was invited by the Russian Orthodox Church, which next year celebrates the 1,000th anniversary of Christianity in Russia. Negotiations have been under way for some time to make it possible for the pope to attend the celebrations.

"The party is very interested in our trip," said Betto, a frequent visitor to Nicaragua and Cuba who has also made several recent trips to Moscow.

"Our task is to combat misconceptions in socialist countries caused by the incorrect interpretation of Marx's famous phrase that religion is the opium of the people. It only becomes opium when when it sides with oppression and injustice," Betto said in an interview.

"In socialist countries we need to show how Latin America has a pioneering role in the construction of future socialist societies with integration of Christians," he said.

Though Boff has been advised by Brazilian bishops to avoid all sensitive issues during his trip, he said before leaving that he would study what positive aspects of Marxist society could be applied to Brazil.

After his punishment was lifted last year, Boff promised to lead the church toward "discovering the mechanisms that generate poverty -- underdevelopment, oppression and tyranny. We recognize the assistance of popular movements like trade unions or new parties that carry the flag of liberation."

Liberation theology emphasizes the grass-roots organization of the faithful poor in the struggle to achieve justice.

Economic instability seems to increase the growing militancy of Brazil's Catholic Church, strengthening support for Boff's ideas. The council of 360 bishops led by Dom Luciano Mendes de Almeida recently expressed alarm to President Jose Sarney over prospects for recession caused by recent adjustments. Sarney admitted "the social and economic situation is so difficult the people are quite right not to be optimistic."