Pope John Paul II's plans for the Roman Catholic Church are still cloaked in mystery a month after he took office. This has led many observers here to conclude not only that change is on its way but that when it does come it is likely to be substantial, sweeping and possibly even traumatic.

The mystery derives primarily from the pope's decision to treat the question of key appointments to the Roman Curia, the Vatican's central administration, differently - and more cautiously - than did his immediate predecessors.

The structure of the centuries-old Curia and the mentality of the men who run it will be considered a key indication of the shape, nature and goals of Pope John pual's pontificate which, since he is only 58 years old, could be a long one.

What happens to the Curia, a body described by its critics as narrow-minded, conservative and cynical, will have a profound effect on the often authoritarian and conservative Congregation for the Doctrine of the Faith, formerly called the Holy Office. It will also be crucial to the final implementation of the reforms worked out by the controversial 1962-1965 second Vatican Council.

For 15 years the church has been divided over those reforms. But since taking office John Paul has reaffirmed his commitment to Vatican II, an event he recently described as a "milestone in church history."

At the same time, the new pope has underlined the importance of the top posts in the Curia by breaking with recent tradition and putting off permanent appointments until his ideas mature about how and where he wants to take the church.

Known as a man with a mind of his own, the former cardinal and archbishop of Krakow, Poland, is a foreigner with none of the Curia experience Pope Paul VI had when elected in 1963. But unlike his immediate pre-in 1963. But unlike his immediate predecessor. John Paul I, who also lacked such experience, he appears unwilling to risk boxing himself in before acquiring real familiarity with the Curia and the men who run it.

Three days after he was elected on Aug. 23, John Pual I formally reappointed all the key Vatican officials and asked them to stay on and finish out the five-year terms to which they had been appointed by Pope Paul.

The new pope has chosen a different tactic. With the exception of the secretary of state, French Cardinal Jean Villot, he has made no formal reappointments.The only indication that the top 18 Vatican officials have been asked to stay on temporarily came in the use of their former titles in announcements that John Paul had met with each of them.

A key to the new pope's "wait-and-see" attitude can be found in the letter he wrote to Cardinal Villot on Oct. 25, 10 days after his election. In it he asked the 37-year-old secretary of state to stay on as a head of the Curia's power center, a position he has held since May, 1969, "during the first part of our ponfifcate."

The pope also used a Latin phrase that means "until other provisions have been made." According to an Italian priest close to Villot, the secretary of state has told friends he expects to stay on only for about five months.

"We can assume the others were told more or less the same things," one Curia official said. He added, "The new pope clearly wants his own team. He is simply taking his time in forming one."

Some progressive Curia members have expressed disappointment over the pope's decision to put off even minimal changes, such as the replacement of two cardinals over 75, the age when resignations must be offered.

But one American Jesuit said, "His decision to wait is really very courageous. He doesn't know the ins and outs of the Curia and he obviously doesn't want to risk making any mistakes." This priest added that the U.S. president has more than two months between his election in early November and his inauguration in late January to form his Cabinet.

Dating back to the 12th Century, the Roman Curia was once a small body of cardinals who met regularly under the chairmanship of the pope. Today it has mushroomed into a network of offices within and around St. Peter's Square, where some two-thirds of the Vatican's 3,000 employes make most of the key decisions regarding Roman Catholics and their clerics.

Curia theologians, working under papal direction, rewrite and revise papal documents, censor theological works, define principles of morality and faith, examine the backgrounds of potential bishops and oversee the rituals of worship.

They scrutinize candidates for sainthood and deal with controversies among clerics and petitions by priests for release from their holy vows.

Currently carried out by layers of congregations, secretariats, courts and commissions, under the direction of the powerful secretariat of state, the responsibilities concentrated in Curia hands have over the centuries created a "Curialist" mentality summed up in a traditional Latin phrase meaning "Rome has spoken."

If, however, Pope John Paul II continues to stress his pastoral role as bishop of Rome and carries through on his recently expressed desire to play on active role in the affairs of Rome's 296 parishes, this dominating role of the Curia could be downplayed and the power of local bishops and of the synod of bishops could increase.

The result, says Vatican historian Giancarlo Zizzola, could be traumatic. He estimates that if the pope moves in this direction, the Curia staff would be sharply reduced. "For the first time, the Vatican would have to cope with theological unemployment," he adds.

In a sweeping 1967 reforms, Pope Padl tried both to streamline the Curia and to internationalize it. He was seeking to meet the complaints of progressives that in its service to the papacy, the Curia often ntglested the needs of the church as a whole.

Pope Paul introduced some significant changes - such as a five-year limit on appointments, the 75-year-old resignation requirement and a continuing pastoral obligation for Curia members - and at the top levels made substantial strides in internationalization. But many think some of his moves have backfired. Rather than implementing, Vatican II's guidelines for decentralization, an increased role for bishops, and a smaller Curia. Paul's reorganization led to a top-heavy administration concentrating considerable power in the hands of the secretariat of state and the pope.

Between 1961 and 1977 the number of Vatican employes soared from 1,323 to more than 3,100. Although the number of non-Italians has more than tripled, the "curialist" mentality reportedly has turned out to be hard to eliminate.

With the strongly Italian low-and mid-level Curia echelons still smarting from the election a month ago of a foreign pope, the new pontiff is widely expected to choose an Italian cardinal as his next secretary of state. Beyond that, however, his plans are unknown. While observers speculate on his many options, they appear to agree that the basic decision will be simple.

Says Zizzola, "At this point the choice is really between containing and controlling the post-Vatican II spiritual revolution as Pope Paul did, or giving it free reign and letting it reach its natural conslusions." The fate of The Roman Curia hangs in the balance.