In the 20-month-old investigation of the shooting of Pope John Paul II, magistrate Ilario Martella was the man who advanced the alleged "Bulgarian connection" from the realm of journalistic speculation to one of criminal charges.

Now much of the world would like to see the evidence supporting the Italian judge's arrest of Bulgarian national airline officer Sergei Antonov on suspicion of "active complicity" in the attempted assassination.

Legal secrecy rules here, however, permit Martella to keep his case under wraps until he seeks to indict the Bulgarian. As a result, the credibility of the charges for the present rests largely on whether to believe the magistrate.

With a potentially major East-West issue hanging in the balance, a dozen judicial sources ranging from court journalists to the defense attorney said in interviews that they were satisfied that Martella deserves his reputation for caution, hard work and avoiding publicity or political intrigue.

"The man is deliberate, methodical and tireless," one veteran courtroom reporter said. "If he is keeping Antonov in jail, it has to be because he feels he has incriminating evidence that outweighs the alibis presented by the defense lawyers."

The papal shooting investigation has attracted international attention primarily because of the hypothesis that Bulgarian intelligence agents may have helped convicted assailant Mehmet Ali Agca, a Turkish terrorist, to plan the attempt. Bulgaria is a loyal ally of the Soviet Union, and, in this view, the Kremlin hoped to silence John Paul's support for the independent trade union Solidarity in his native Poland.

Some in Italian judicial circles fault the shortish, conservatively dressed Martella, 48, for being naive, inexperienced in terrorism cases and stronger in diligence than intuitive brilliance.

This criticism was echoed by Antonov's defender, Giuseppe Consolo, who said that Martella is not "street smart" and does not realize that the evidence against Antonov may be trumped-up. But even the defender conceded that his legal rival "is really top rate, a first-class person, extremely intelligent and totally honest."

Martella has been at the center of a big case before. As a magistrate in the Rome attorney general's office in 1975, he launched the judicial expose of Italy's Lockheed payoffs scandal by filing corruption charges that eventually led to a jail sentence for an Italian defense minister. The case marked the first prosecution of Italian government ministers since World War II, and Martella was credited with handling the case successfully despite a highly politicized atmosphere.

This time the magistrate has taken time-consuming care to build his case against Antonov, two former Bulgarian Embassy employes who were implicated after leaving Italy, and four Turks who also have been charged with helping Agca.

After being appointed examining magistrate for the papal shooting in the fall of 1981, Martella traveled quietly to Turkey, West Germany, Austria, Switzerland and the United States. Judging from the list of Martella's suspects, Agca allegedly was involved in a shadowy network of Turkish emigres and smugglers in Western Europe, and Bulgarian government employes in Rome.

Judicial and political officials now have confirmed that Agca, who is serving a life sentence in Italy for his crime, began cooperating with Martella in June 1982 and picked out photographs of the Bulgarian suspects from a specially compiled album in September.

Later that month Martella said that he could not rule out the possibility of a Soviet Bloc plot, and on Nov. 25 he ordered the arrest of Antonov. Since then, he has rejected the defense lawyers' claim that the Bulgarian should be released because of "total lack of evidence."

In the Italian government's first official comments on the case, Defense Minister Lelio Lagorio told parliament in December that the shooting was "an act of war." A senior Italian government official said recently that Lagorio's comments constituted "a warning" to the Soviets that certain limits must be respected in international relations.

Although Italian judges and other courtroom officials often disregard judicial secrecy, leaks have been very rare in this case. Reporters have had to rely on hints from the defense lawyers and, more frequently, on news trickling back from Turkey.

"Martella is so reserved that he hasn't even told me everything he knows," a magistrate in another case with a Bulgarian angle recently confided to an Italian reporter. Another Italian courtroom journalist who occasionally has seen Martella socially says that he is inveterately close-mouthed, a firm believer in a free press but equally convinced that the latter's professional interests are often diametrically opposed to his.

Unlike some Italian judges, Martella is politically unaffiliated. He has shunned membership in any of the three politically hued magistrates' unions.

"He appears to be totally incorruptible," a high-ranking Communist Party member said recently of Martella. The Communist said he personally was skeptical about the theory that the Bulgarian or Soviet governments had plotted to kill the pope, but on the basis of Martella's reputation he thought it likely that there was evidence that Antonov and Agca had had contacts of some sort.

Martella, who is self-effacing in interviews, appears genuinely to dislike the attention that he has attracted because of the investigation.

"It upsets me profoundly that a magistrate should become famous because he is handling a dramatic case," he recently told a friend. "Putting oneself in the limelight is for movie stars, not for judges. If I were to do that, it would be a real betrayal of my personality and my beliefs."

Now the unwilling victim of reporters' attention as well as a presumed target for international terrorists, Martella is considered to need special measures to ensure his security. He now has a round-the-clock police guard at his home, a bulletproof car for commuting and a pair of uniformed paramilitary police guards at the door of his office.