Paintings of popes and cardinals, a veritable history of the Catholic Church in recent centuries, gaze at one another across the cavernous anterooms of the apostolic delegate's residence on Embassy Row. But 11 months after he assumed the papacy, a portrait of John Paul II sits on the floor of the main hall behind the vast double doors, waiting to be hung. It's hard to find a spot where this new pope -- who appears so young and smart and tough compared to his many dessicated predecessors -- will not look out of place.

Archbishop Jean Jadot walks through the hall. He is the delegate: the pope's man in Washington. Laughing easily and talking with unpretentious intelligence in his soft Belgain accent, he doesn't fit amid all this sterile ostentation, either. His is a mansion built for other men and other times.

He isn't there very much.

In the world of diplomacy, the title of apostolic delegate is unique, perhaps because the idea of diplomats' representing both a spiritual state and, to a much lesser extent, a physical state -- the 108.7 acres of Vatican City -- is hard to grapple with.

In about 90 countries, there are official diplomatic representatives of the Roman Catholic Church who are recognized as ambassadors. Nuncios, they are called, or pronuncios. But in a country such as ours, where the whole notion of a state that is a church runs counter to our founding principles, no such ambassadors are recognized.

Enter the apostolic delegate.

Like a nuncio, he's expected to watch over and work with the Catholic Church in the country he's sent to. Unlike a nuncio, he's supposed to have no official diplomatic functions whatsoever.

But nothing is so absolute, and the role of a delegate in the church and with the state is likely to be one of infinite subtleties, depending often as much on the man as on his title.

"I don't think we are diplomats at all," Jadot said recently, sitting in one of the few hospitable corners of the official residence on Massachusetts Avenue. "We are priests . . . and, in some situations, we have to fulfill diplomatic duties."

He smiles slightly as he says this. On a table nearby are a diplomat's usual photographs of himself with President Carter, former President Ford and Vice President Mondale, whose house is across the street. Above the cleric's collar, Jadot's face is 69 years old but full of vitality. His eyes, behind fashionable silver rimmed glasses, are intent on his listener.

"I am," he said, "a kind of instrument between the heart of the church and the periphery of the church in the United States."

If that is so, then Jadot is "a kind of instrument" that the Catholic Church -- and the United States -- had never seen before he arrived here six years ago.

Most nuncios and apostolic delegates are products of the Pontificia Accademia Ecclesiastica. For nearly 280 years this institution at the Vatican has been training young clergymen -- until recently almost all of them Italians -- to enter the papal diplomatic corps.

Until Vatican II they were, quite literally, the pope's men in whatever country they were stationed, part of a direct chain of command from Rome through the cardinals and bishops right down to village priests. Many grew accustomed to authoritarian power in countries with large Catholic populations, some of which had direct involvement between the Church and state. Even in some places where such powers did not exist for the nuncios, they were accorded prestige and ceremonial standing as the automatic deans of the diplomatic corps, simply as a matter of tradition.

As one American priest familiar with the inner workings of Vatican diplomacy succinctly put it, "Jadot did not come in that way."

He is, as far as the old-boy network of the Vatican's Curia is concerned, an outsider. First of all, he is not Italian. Born in Brussels in 1909, he is the son of a civil engineer who spent much of his time in China.

"We were eight children at home," Jadot said, his smile expanding across his face. "I have seven sisters. I'm the only boy, and laughing from time to time I say I became a priest because I saw too many girls."

While other nuncios-to-be were studying the Accademia, Jadot was an assistant pastor at a church in Brussels and the chaplain of the Catholic student organization there. In the 1950s he was a chaplain with the Belgian forces in the Congo, then returned to Brussels to head the influential Society for the Propagation of the Faith there in the early '60s.

He had spent much of his life traveling to foreign lands on missionary work, but Jadot had none of the Vatican diplomat's usual credentials when suddenly, in 1968, Pope Paul VI placed him in the diplomatic corps as a pronuncio. With the post went the title of archbishop.

Even about this exalted appellation Jadot reserves a sense of humor. Members of the Curia and especially the diplomatic corps are often bishops without any real diocese. As Jadot was walking through the courtyard of the apostolic delegation the other day, he introduced a visitor to his English springer spaniel, Zuri, who was busily licking his master's hand. The visitor asked about the dog's name.

"I am Archbishop of 'Zuri,' " Jadot laughed.

Jadot's first posts as a Vatican diplomat were in Southeast Asia and Africa. The in 1973 -- to his surprise, he says -- he was made apostolic delegate in the United States.

"Really, I don't know why I've been sent here," he said recently. "I was asked if I was willing to go. I said yes. But they never told me why."

Some observers believe this account of his appointment is a little disingenuous. "Paul VI must have had a very strong feeling about this man," said an American priest familiar with Jadot and the Vatican hierarchy. "We had had some disasters for apostolic delegates: [Egido] Vagnozzi, who was here in the '50s, started a witch-hunt against theologians. [Luigi] Raimondi was a little less authoritarian, but he had a tendency to think in terms of damage limitation as far as the church was concerned.

"Then all of a sudden we get this very charismatic, engaging, highly intelligent guy. Jadot has enormous, insatiable intellectual appetites. He's had an enormous impact because of his personality, no doubt about it. He's no Andy Young, but there are lots of nuncios that don't have anything like the effect he's had."

At the time there was speculation that Jadot was sent here to "shake up" the American Catholic Church. It was fast losing large numbers of members, many of whom felt profoundly alienated from its hierarchy and confused by its doctrines.

Whatever the reasons for his coming here, Jadot, through his influence on the selection of new bishops, has been at least indirectly responsible for some of the most significant developments in his church here during the 1970s. The bishops' appointments are channeled through his office, and it's up to Jadot to advise the Vatican on the feelings and needs of the parishioners the bishops will serve. More than a third of the active Catholic hierarchy in the United States has been appointed since Jadot arrived, and they have been, as a rule, younger, more inclined to community work and in many cases more liberal than their immediate predecessors.

Jadot naturally plays down the idea that he has put his own mark on America's church.

"I know that rumor is going around," he said slowly, "but I am not too convinced, on consideration. It is not for me to say that it is nothing, but I think it is overdone. In fact it is mostly the work of Pope John XXIII and Pope Paul VI insisting on the necessity of appointing, as bishops, priests who relate easily to people -- who are people-oriented -- more than managers or priests who are oriented toward organization. At one time I would say we needed those bishops . . . building bishops" -- meaning, in many cases, literally builders of churches, schools and cathedrals. "Today we are one step further and we need, I feel very strongly, shepherds of the church who are caring for people."

Jadot's own sense of people and his ability to work with them suits him peculiarly well to the new, less autocratic role played by papal representatives since the Second Vatican council ended in 1965.

As Jadot describes the change, "Before Vatican II each bishop was relating only to home" -- that is, to Rome. "Now in every country you have a bishop's conference which is in charge of coordination, common work, exchange."

Though he is no longer part of a direct chain of command, the delegate plays a vital role in this coordination, this generation of ideas and reevaluation of old relationships. Such things are totally dependent on the understanding between Rome and the church in the rest of the world. Jadot finds himself with the task of explaining the Americans -- some of the church's more headstrong constituents -- to the Vatican, and the Roman hierarchy to Americans.

"There is a big difference between knowing something, knowing a situation, and understanding a situation," said Jadot. "To report facts -- that's not the point. When you have to give understanding, that needs much deeper insights. This I suppose, depends on man to man. For me to go somewhere and to spend only one hour, I feel much more that I understand the situation than 20 pages of a report."

Through such contacts, he feels, he is able to give Rome an evaluation of whether troubles in the American church are localized or widespread, and how deeply they are ingrained in American life. Jadot travels incesantly, though he said recently that he's trying to slow down.

"I am trying now to get used to a pace of one trip a week and trying always to have about five, six days between trips. You see this week I was in Boston, last week I was in Jackson, Mississippi. The week before I was in -- I forget where I was -- Joliet, Illinois, I think. Next week I am in Pennsylvania. Then comes the visit of the pope. And even after that I have to go to Texas."

Jadot is clearly fond of the American he has found and the church he has perhaps, helped to shape here. But he is troubled by it as well. At its core, he suggests, are deep and profoundly unsettling problems -- "problems of principles, the philosophy of life."

"Don't you note," asked Jadot, "that there is such confusion in so many minds that they don't know what more to think about something? They are really lost . . .

"For instance, let's take the problem of marriage. I don't speak here of philosophy, but just looking at personal problems. What is the value of fidelity? You make a commitment for life and then after five years it seems, well, you don't want it. But that is not only a problem of marriage, it is much deeper."

Such problems, Jadot feels, can only be resolved by strong intellectual and doctrinal leaders. These are the kinds of men he looks for as bishops, and what he believes he has found in his own superior, Pope John Paul II.

"He is expressing very strongly some very strong convictions," said Jadot, his voice full of irrepressible enthusiasm, "and if you don't like it, well, he comes back on. He's convinced."

Perhaps it's because of the obvious pleasure he takes in the direction the church now appears to be going that Jadot answered a reporter as he did last spring when asked if, on his travels around the country, he acts as the "eyes and ears" of the Vatican.

"I am looking at myself," said Jadot, "much more as the heart of the Holy Father than his eyes and ears."