Beyond inheriting a spiritual kingdom of 600 million Roman Catholics, the successor to Pope Paul VI will also take charge of an international financial empire that has been valued at anywhere from hundreds of millions of dollars to as high as $12 billion.

The actual value of the Vatican's holdings is a closely guarded secret.

But the Roman Catholic church's empire includes hundreds of historic churches, a vast collection of gold and silver crucifixes, gems and art objects, stock in companies the like of General Motors. Shell, Gulf Oil and the New York Central Railroad, chunks of prime real estate, interest in scores of Italian concerns from banks to real estate developers, and a collection of priceless bibles.

All this is monitored, if not managed, from 108.7 acres of valuable land that constitutes the Vatican - the smallest sovereign state in the world.

A decade ago, a relative of one or Rome's more prominent cardinals privately estimated that Vatican investments in the United States alone ran somewhere above $65 million.

The Vatican has been tied financially to such titans of the investment world as the Rothchilds, who have historically bailed out fiscally strapped popes, Hambros Bank in Britain, the Credit Suisse in Zurich and London, and in New York, Chase Manhattan and Bankers Trust.

Through the 1960s the church controlled the international real estate giant, the Societa Generale Immobiliare, holding 15 percent of the outstanding shares. Immobiliare built the Watergate complex in Washington, the Cavaliere Hilton in Rome, Montreal's Stock Exchange Tower and the prestigious Pan American Building on Paris' Champs Elysees.

While the church's interest in Immobiliare has reportedly dropped to 5 percent, reflecting a move to broaden investments from largely Italian concerns to a wider spread on the world market, its influence can still be felt.

In 1929, the Italian government signed a treaty with the Pope creating the present Vatican state and giving it $90 million in reparations for land taken away from the pope's domain. The money has since been invested well.

The Vatican bought into banks like the Banco di Roma, created new companies, took over the Italian gas conglomerate Italgas, and controlled at least two powerful insurance companies, General Insurance and RAS.

Sometimes the church's financial dealings have been curiously amusing. A papal representative who concluded an agreement with Hungary in 1964 persuaded the Hungarian government to buy all its bathroom porcelain from a Vatican-controlled company.

Other times, the church's investments seem to conflict oddly with its policies. The Vatican, for instance, has long held a substantial interest in the Instituto Farmacologico Serono, an early producer of birth control pills despite the church's proscription against contraceptives.

Despite the impressive statistics, Vatican investments have not always been a success.

The church was forced to sell a faltering pasta company serveral years ago and reportedly lost millions of dollars when the financial empire of Sicilian banker Michele Sindona collapsed.

But the money is still there. The $12 billion estimate is probably absurdly high.

A more reliable estimate has been quoted from Massimo Spada, a former Vatican financial consultant who inheritied from his father a position among the small but influencial Itlian aristocracy of laymen who represent the Vatican on corporate boards. The figure he mentioned in 1975 was $560 million.

Spending by the church, whether it is controlled by Rome or by one of its bishops, can have a major economic impact.

The Vatican controlled the engineering firm that built the San Bernardo tunnel between Italy and Switzeriand, affecting both jobs and commerce.

It was estimated in 1964 that construction spending by the archdiocese a year, and in 1960 Fortune magazine pegged the annual receipts of that archdiocese at $150 million.

Financial dealings are handled primarily by the Institute for Religious Works and several lesser official bodies. The principal manager of the church's assets is Egidio Cardinal Vagnozzi, but prominent among his aides is Bighop Paul Marcinkus, an American from Chicago with a reputation for financial shrewdness.

The move from investiment on an Italian base to a more international portfolio is largely attributed to them, based partly on their reaction to the failure of several Italian ventures and partly on an Italian ruling that forces the church to pay tax on some dividends, despite its tax-exempt status.

Certainly part of the Vatican's problems lie in the liquidity of its assets. While stocks can be converted to cash quickly, land cannot, particularly when there is a church on it.

To what degree this makes the Vatican depend on the "Peter's pence" - the annual voluntary contributions by parishioners around the world to fund Vatican operations - is uncertain, but clearly the pence plays an important role: New York alone is reported to contribute more than $1 million annually.

The Vatican budget has added to problems incurred in depressed world financial markets in the last few years.

In 1975, Pope Paul rejected a budget proposal because of an estimated $6-million deficit, and subsequently set up a study commission that recommended sharp staff cutbacks.

Other than cutting back on costs of the Curia - the church's central governing body - there is little the Vatican can do unless its stock portfolio begins to yield more. Other expenses, such as missionary work and charities can ill afford cuts.

The next pope will have to deal with these very temporal issues in a manner that, if not infallible, is at least prudent.

For if a money crunch ultimately comes, as some say it will, Rome's financial managers may well find their assets include a number of distinctly nonliquid items, from the papal palace to Michelangelo's priceless Pieta.

And as a ranking priest once put it, "You can't sell the Sistine Chapel."*