Among the covey of black-robed bishops clustered around Pope John Paul II on his American trip is a prelate conspicuous for his plain black priests suit and Roman clerical collar; Bishop Paul Casimir Marcinkus from Cicero, Ill.

The highest ranking American in the Vatican bureaucracy, Marcinkus is known as the Pope's advance man and bodyguard, a role he began with Pope Paul VI. But his regular job is as head of the Vatican bank, called the Institute for Religious Works.

In 1970, Marcinkus was credited with helping to fend off a deranged artist who wounded Pope Paul with a knife during the pontiff's visit to the Philippines. Because of his enthusiasm in keeping crowds away from Paul, he was once nicknamed "II Gorilla."

Marcinkus coordinated this papal tour, visiting the places the pope was to go, checking the facilities and neighborhoods the pope was to pass through. He stays close to the pope as he moves from one place to another, hovering near him protectively but unobtrusively.

The son of a window washer, Marcinkus, 57, is of Lithuanian descent, an ethnic identity that is reported to have aided his relationship with the Polish pope. Tall and athletic, he has lived in Rome for at least two decades as he moved up the Vatican hierarchy. He once tried to form a Vatican golf team, but no one was good enough to play with him.

His facility in dealing with the practical, worldly details of a papal journey apparently impressed Pope Paul, who elevated him to bishop and, despite his admitted lack of financial experience, appointed him secretary and then president of the Institute for Religious Works 10 years ago.

His prior financial experience was limited to "handling the Sunday collection," he told the National Catholic Reporter in 1975.

Speculation that he was going to be replaced surfaced in the Italian press after Pope Paul's death, but John Paul reconfirmed his appointment. Criticism of his handling of the bank's money mounted after revelations that Sicilian financier Michele Sindona, brought in to advise Marcinkus, subsequently cost the Vatican millions of dollars when his empire fell apart.

Vatican finances are among the most secretive in the world, and Marcinkus has upheld that secrecy. Reputed to have billions of dollars and other wealth in real estate holdings, the Vatican is exempt from scrutiny because it is not covered by Italian laws.

Sindona's involvement came in the late '60s when Pope Paul decided to sell the Vatifan's controlling interest in the Societa Generale Immobiliare, the company that built the Watergate complex and the Rome Hilton, among other enterprises. According to a series of articles on Vatican finances that appeared in The London Sunday Times in 1975, the church's holdings in that company amounted to about $350 million.