Roman Catholic leaders here fear that 15 years of liturgical reform may be jeopardized by this week's unexpected Vatican directive authorizing a limited return to the old-style Tridentine Latin mass.

The use of the old Tridentine mass, said in Latin, could "bring out old divisions where divisions have already been healed," said the Rev. John Gurriri, director of the liturgy secretariat of the National Conference of Catholic Bishops.

"I see it as a terrible move," said the Rev. Gerard Austin, chairman of the Catholic University theology department and a liturgy expert. Austin predicted that the new ruling will "undermine the changes in the liturgy."

Austin said the problem is not with the use of Latin. "It's a question of using the revised liturgy of the church, which was done after years of study and consideration," he said.

At issue is the form of the mass itself, which, according to Gurriri, changed the role of lay Catholics from spectators to participants.

The current rite, called for by the Second Vatican Council and issued by Pope Paul VI in 1969, "calls for participation in the church's life by all the people, not just priests and bishops, so the Sunday assembly in the parish could become the focus of church life," Gurriri said.

As a result, the liturgy is now spoken in modern languages, and the altar has been turned around so the priest faces the people. Lay readers and commentators take part and lay persons assist in distribution of holy communion.

"By reforming the liturgy, we reformed the whole church," Gurriri said.

Gurriri said that after 15 years with the new mass, the old formula "is not a living liturgy anymore. It belongs in another period of time."

Contrary to popular belief, the use of the Latin mass was not outlawed by the 1969 revision.

Locally, Latin masses, using the authorized 1969 rite, are celebrated each Sunday at both the Shrine of the Immaculate Conception and St. Matthew's Cathedral. A spokesman for the cathedral said that as many as 500 persons from throughout the metropolitan area come for the Latin mass at 10 a.m. each Sunday.

For more than a decade, the Vatican has sought to bring into line the rebel French Archbishop Marcel Lefebvre and his movement using the Tridentine mass.

Lefebvre and his followers reject the authority of the Second Vatican Council, and some in the movement do not acknowledge the legitimacy of the election of the last two popes. Lefebvre has continued to function from headquarters in Switzerland despite suspension of his priestly faculties by Pope Paul VI.

There is widespread speculation that Monday's liturgy change may have been an olive branch from the current Vatican administration to the traditionalist movement. "I think it was the pope's way of trying to pull some people away from certain leaders like Lefebvre," Gurriri said.

Under Monday's directive, the Tridentine mass may be used with the permission of the local bishop, but participants must declare publicly that they recognize the "doctrinal legitimacy and exactness" of the 1969 rite -- a stipulation that the Lefebvrists are not expected to accept.

While there were reportedly rumors about such a move more than a year ago, Monday's directive "came out of left field" for Catholic leaders in this country, Gurriri said.

In contrast to the Vatican's usual careful use of channels to communicate such matters to the appropriate leaders in its churches around the world, Gurriri said he first learned about the latest directive "when I opened The Post at breakfast.

"I nearly choked on my coffee," he said.