The acting archbishop of San Salvador, who until recently supported an opening by the Roman Catholic Church to El Salvador's leftist forces, now declares that the left has lost its struggle against the government and that the influential church here must stay in a neutral, centrist position.

The Rev. Arturo Rivera y Damas said in his sermon today, "We are pleased that [President Jose Napoleon] Duarte is convinced that a dialogue [with the leftists] is necessary. We encourage him in this since it is neccessary to make the force of terrorism disappear."

No institution has suffered more or changed more in the Salvadoran civil war than the church.

Three American nuns and a lay worker were killed in December. The humble but powerfully influential archbishop, Oscar A. Romero, was assassinated while saying mass a year ago. Ten other priests have been slain since 1977. Virtually all Jesuits have been driven out of the country. The cathedral, the churches and their schools have been seized by leftists and bombed by rightists.

Most of the persecution of the church clearly has come from the extreme right and from its allies in the military. The government has left "much to be desired" in its respect for human rights and has delayed investigations into crimes against church people, said Rivera, the apostolic administrator and acting archbishop, in an interview.

It was such acts that led Romero, a political conservative, to sympathize publicly with the left and the poor who followed it in the years before he was killed. But Rivera said the guerrillas and their allies are also to be condemned for their actions and if the church is to be with the people, then it must be in a neutral center.

A year ago Rivera was the most steadfast, sometimes the only supporter of Romero within the church hierarchy. But the situation of the country has changed dramatically since then, and in the last month or so, Rivera has increasingly indicated in public statements that he now views the government with more sympathy.

"These are very different moments," said Rivera. "In those that Romero lived, there was hope in the liberation movements. There was a great sympathy on the part of the people. Later these movements were losing credibility and popular support.

"I don't think they lost it only because of government repression," Rivera said. "They lost this support because the people say that they tended toward the conquest of power for its own sake and not toward satisfying the hopes of the people."

This point of view has set Rivera at odds with some of the more radical priests within the country and with Catholic leaders in much of the rest of the world who oppose the U.S.-backed Salvadoran government. Some priests here have gone so far as to take arms alongside the guerrillas. But Rivera says they are deluded.

"In general the international information is pretty partial," Rivera said. "They either don't know or they try to hide things."

Rivera cited the example of the church legal aid service here that recently reported military searches and abuses in church refugee camps but not the fact that guerrilla bombs were found there.

"I'm very unhappy with the way they have been acting because it wasn't a service of the truth," said Rivera.

He said the majority of the refugees in the country are fleeing not the government but the guerrillas. "It was not only because of government repression that people drew away from the guerrillas but also because of their demonstation of radical leftist tendencies and for their sabotages."

Leftist guerrillas frequently have attacked public transportation and places of business vital to most Salvadorans.

Rivera still criticizes the right. In his sermon today he accused rightists of "frustrating attempts to introduce social reforms. This is one of the reasons that a majority of the people have remained unconvinced of the current government's sincerity." He declared the extreme right to be "the true cause of the country's crisis.

At the same time, he added that the guerrillas "have made violence and loyalty to Marxism their watchword, and because of that the majority have turned their backs on them."

The church supports about 6,000 refugees who are connected in one way or another with the guerrillas -- mostly wives and children -- and need protection from the government, Rivera said in the interview, while 50,000 to 60,000 have fled the zones of the country occupied by the left.

"When I came to the archbishopric I tried to locate myself in the center, taking a critical distance as much from the government junta as from the Democratic Revolutionary Front. But also I've tried to point out, with emphasis, that the church needs to concern itself with justice and that this justice be realized among our people -- that our peace ought not to be a peace of the cemeteries but one that would try to confront the roots that cause there to be no peace."

A staunch supporter of the reform process initiated by the government and aided by the United States under the Carter administration, Rivera said, "If instead of going ahead with the reforms the government steps backward, I'm sure that you would have anew the explosion of desperation because of the unbearable situation. But that would be in the case that the reforms don't continue. They are responses to real problems and therefore the causes of injustice are being resolved, although partially, and this certainly would play a part in lowering popular tensions."

Although he has played a serious role in attempting to initiate dialogue and negotiations between the left and the government, Rivera says a solution may eventually be found within the current regime.

"I am persuaded that the insurrection, at least in the short run, will not succeed," said Rivera. "I consider that in the center there is a great quantity of people who have not made a choice. It's not that they are with the junta, but they considerr it as a necessary evil, or at least that it creates a security and a certain guarantee."