By circumstance and by choice, Pope John Paul II is standing at the center of the whole complex of East-West relationships, not to mention Ronald Reagan's thorniest foreign policy problems.

Because of the plot against his life, the Bulgarian connection that the Italian police have established and the possible, although far from proven involvement of Yuri V. Andropov, the new Soviet leader and erstwhile head of the KGB, the pope could become the glass through which the free world looks at Moscow.

The president has been preternaturally restrained in his comments about an unfinished story that so far reinforces a thousandfold his stated views of the Soviets as perfidious and brutal people who stop at nothing to divide and conquer the world.

The monstrous suspicions engendered by the reports would constitute a perfect alibi for halting the disarmament negotiations he so reluctantly opened in Geneva. The explanation that he wishes to preserve detente is hardly convincing, in view of his recent attempts, through pipeline sanctions, to discourage it. Not all public figures are so reticent. Sen. Alfonse M. D'Amato (R-N.Y.) is storming around Rome shouting "J'accuse" at the CIA for ineptitude in investigating the enormous crime, about which, he rages, he gave them advance information.

Adding to the riddle is the fact that the CIA has been charged with leaking accounts that pooh-pooh Bulgarian and KGB involvement. Why? Is it covering for its failures on the case? Does it have better information? If so, why is it not divulged?

The pope also is a pivotal figure in the matter most galling to the administration--the U.S. bishops' controversial anti-nuclear pastoral letter.

According to columnists Evans and Novak, the White House dispatched Gen. Vernon Walters, a veteran troubleshooter, to the Vatican to importune the pope to rein in his radical clerics, who oppose first use and other nuclear "options" Reagan wishes to retain. Walters indignantly denies the Evans and Novak report, but plainly the administration hoped that the Americans, who were recently summoned to Rome to meet with their dissenting European brother-bishops, were being called on the carpet.

This was not the purpose or the outcome of the bishops' meeting, according to eyewitnesses. Differing opinions were exchanged in an amicable, fraternal spirit. The third draft of their pastoral letter, which the president wishes would never see the light of day, is going forward.

The administration thought at one time that it could circumvent the U.S. bishops, who have taken an adamant position against its policy in Latin America. They had some initial success, but last August, the pope issued a pastoral letter that proved he did not share the administration view of the war in El Salvador as a struggle between East and West with the guerrillas cast as clients and pawns of Moscow and Havana.

He defined it in these terms:

"On one side those who consider armed battle a necessary instrument for obtaining a new social order, and on the other side those resorting to the principles of 'national security' to legitimize brutal repression."

Now the pope will go and see for himself. Early next month he is embarking on a tour of Central American hot spots, El Salvador, Honduras, Guatemala, Nicaragua. His itinerary is a tribute to his courage and also his determination to be taken into account in world affairs.

The State Department hopes that it may be just another of those picturesque pastoral progresses, with the white-clad figure lost in the seas of screaming, yearning humanity that envelop him wherever he goes. But the political potential is too great for comfort.

If the pope, for instance, makes Rivera y Damas the permanent archbishop of El Salvador, it is bad news for the administration. Rivera, who has been "acting" for three years, is a critic of the security forces and an outspoken advocate of "dialogue," that is, negotiations with the guerrillas. His installation would be seen as a papal endorsement of Rivera's approach.

The most the administration could hope for in the pope's visit to Nicaragua is that the pontiff will be preoccupied with ecclesiastical offenses--the presence of disobedient priests in high places in the Sandanista government and the recalcitrant "popular church" which the pope wishes to convert to the Polish virtues of "unity and discipline." But he may find time to speak of "reconciliation" between Nicaragua and its neighbor Honduras, which recently engaged in military maneuvers with forces of the United States, which has never denied a covert campaign against the Marxists in Managua.

His resonant baritone will be heeded. John Paul II, by accident and design, has long since become more than the shepherd of his flock.