Throughout the Cold War the Vatican often exhorted Romans against allowing the Cossacks to arrive in St. Peter's Square." But nine months into Rome's first leftist government. Vatican officials appear to have concluded an uneasy truce with the communist's esconced on Rome's Capitoline Hill.

Last spring Pope Paul warned repeatedly that Marxism and Christianity were incompatible and that a Communist victory in last June's city elections would turn the "sacred city" into "a city without God."

The present calm at the Vatican on the other side of th Tiber River is partly because elsewhere in Rome the Christian Democratic national government owes its survival to continued Communist support.

The Vatican is also used to dealing with Marxists in Communist-ruled Eastern Europe and aside from the cancellation of one traditional municipal mass this week, signs of "godlessness" have been few.

Last June the Communists scored an expected victory over the Christian democrats, winning 35.4 per cent of the vote and 30 seats to the Christian Democrats' 28 in the 80 member city council.

Part of a general trend that had already brought leftist administrations to power in most major Italian cities, the Marxist victory in Rome led to the formation last August of a Communist-dominated three-party city government.

The advent of a "red" capital was greeled exultantly by the local left but was a deep embarrassment to the Christian Democrats and traumatic for the Vatican. For centuries the seat of Roman Catholicism. Rome today still houses hundreds of religious orders and thousands of priests and nuns, and only two years ago played host to millions of Holy Year tourists.

Since taking power, however, the local Communists have been bending over backward to reassure the Roman Catholic world, as one Rome official put it, "that we don't eat Christian babies."

A case in point was the selection by the Communist-dominated city council of Giulio Carlo Argan, a non-politician and non-party member known primarily as a Marxist man of letters as mayor of Rome. Party sources explain that Argan is a more reassuring figures than the city's top Communist, Luigi Petroselli, an old-time apparatchik.

The Pope granted the mayor a private 45-minute audience in January at which Argan said they discussed the city's problems. Later the Pope urged cooperation between city and church officials, indicating a partial thawing of the rigid anticommunism first expressed by Pope Pius XI in a 1937 encyclical.

Padre Bartomoleo Sorge, editor of the semi-official Jesuit monthly, "Civilita Cattolica," says "the Church has concluded it must respect the outcome of a democratic choice."

The church is as hostile as ever to communism, he said, "but given the recent growth of the Italian Communits Party. The Vatican has decided to follow the policy it adopted years ago in Eastern Europe; good relations as long as there is religious freedom and respect for church activities."

Sorge think Rome's Communists are unlikely to create real problems because "they are looking ahead." With many practicing Catholics among their 12 million voters, the Italian Communists are eager for a long-term rapproachement with the country's many Catholic believers - a policy they call the "historic compromise."

The church has been largely unresponsive to this concept, but in recent years has favored a courteous and constructive dialogue rather than the fire and brim stone pronouncements of earlier years.

After 257 days in office, the communists have produced few signs of the revolution in city management hoped for by those who blamed the city's chaotic condition on the Christian Democrats, the Vatican's traditional political ally here.

Rome, the nation's captial with a population of 3.5 million, has nearly empty coffers, $6 billion in debts, poor social service inadequate transport and large slums.

Like Naples and Turin, where the Communists and their allies took over in mid 1975. Rome provides the Communists with a greater challenge than smaller cities like Bologna in northern Italy, where they won a reputation for efficient government.

According to Mayor Argan, the lain problem facing Rome's new administration is the unchecked urban sprawl caused by years of systematic disregard for city zoning laws. Rampant real estate speculation "has got to be stopped," he said, adding that the city's historic center "must be protected from greedy landlord who illegally convert Rome's old building into elegant residences the original inhabitants simply cannot afford."

The city has already started cracking down on unlicensed builders. It also has published the names of Rome's biggest tax evaders, decentralized the sanitation department and oiled the wheels of the city's Byzantine bureaucracy. So far, however, few practical steps to ease Rome's pressing problems have been taken.

"The most we've been able to do is to cut back on wasteful expenditures that make a bad impression," says Mayor Argan, who immediately sold 400 city limousines and cancelled several ceremonies and exhibitions.

"We know this is like Gianni Agnelli trying to get his Fiat car company out of the red by giving up cigarettes," quipped Argan a wizened grav-haired man with glasses, "but public opinion does count."

The Communists insist that they don't mind the challenge of Rome's difficulties because they want Italians to see them as ordinary people. They nevertheless are optimistic that during their five-year term they will be able to make a dent in the city's problems.

"We've got a lot boiling in the pot," said one highly placed Communist official in response to Christian Democratic charges that the city's Marxist officials are "too bureaucratic" and "lack imagination." "If that's the worst they can say about it than I'd say we're doing just fine," he said.