Nearly 200 Cuban Roman Catholic bishops, clergy and lay people gathered this week in Havana for a conference that some have characterized as the most significant Cuban church gathering in a quarter century.

Pope John Paul II sent a special message to the meeting, the first official gathering of the Catholic Church in Cuba since Fidel Castro came to power, offering his blessing and prayers for a "joyous fulfillment" of hopes for lessening of tensions.

The gathering, which concludes Sunday, is the latest in a series of indications that the Castro regime is easing its opposition to organized religion.

When Cuba was declared a socialist state more than a quarter century ago, the government nationalized church schools, expelled foreign priests and placed restrictions on religious worship. At one time, persons identified as believers often were barred from universities and from job promotions.

But a more relaxed climate has developed within the last year. Castro held lengthy conversations a year ago with a delegation of Catholics from the United States and subsequently with the Cuban Catholic hierarchy.

An office of religious affairs was created and in a series of interviews with a leftist Brazilian churchman, Castro detailed his religious views. Published in book form in December, the book is a best seller.

Earlier this year, the leaders of the Cuban Communist Party incorporated into the draft of a new five-year plan a call on all citizens to respect the beliefs of churchgoers.

The draft, which is expected to be approved at a party meeting in December, urges Cubans to honor "the moral integrity of believers" and to avoid practices that could "wound religious sentiments."

"Within the party's policy of encouraging national unity, there is no room for discrimination against believers. Instead, they should be encouraged to participate voluntarily and consciously as citiens and patriots in the construction of socialism," says a draft of the document, which has not been made public but was made available to some diplomatic missions.

Planning for the Cuban Catholic conference in session this week has been under way for several years. The pope sent Cardinal Eduardo Pironio, a Vatican-based Argentinian who heads the Pontifical Council for the Laity, as his personal representative.

In his message, the pope noted "the prolongation of difficult years" Cuban Catholics had experienced, but added hopes that the meeting will "give to the church in Cuba a renewed apostolic enthusiasm."

"May the grace, matured over long and difficult years of prayer, sacrifice and selfless commitment . . . of numerous Catholic Cubans stimulate your work," the pope's message said. "I am sure that in their example of faith, service and charity, you will find inspiration."

As the delegates greeted the pontiff's words with a standing ovation at the opening of the conference Monday, Pironio assured them, "The affection with which you have received the Holy Father's message assures the communion of your Cuban congregation with the universal church."

Archbishop Patrick Flores of San Antonio represented the United States Catholic Church hierarchy.