THE VATICAN Collections: The Papacy and Art," now visiting New York, will never be seen clearly. Venerations fog it. Its objects are surrounded by so many various haloes that the haloes bump.

That length of silken twill once lined the silver reliquary in which the medieval church preserved the sandals of the Christ. That little cross-shaped casket once held a jeweled cross believed to contain portions of His body, His umbilical cord, His foreskin and bits of the True Cross. That covered Silver bowl contained the head of St. Sebastian. Surely these are objects that cannot be judged just as works of art.

The Vatican exhibit, which opens to the public Feb. 26 at the Metropolitan Museum of Art, is a panoply of relics. Not all are of the Catholic Church. Many of the finest are, in fact, pre-Christian. Some are Roman, some Egyptian, some Aztec, some Eturscan. It is the spirit of art history that lends these things their grace.

Consider, for example, the Apollo Belvedere, of all the objects shown perhaps the most renowned. It used to be acknowledged by all educated persons as the greatest of Greek statues. Today its drapes seem prissy, and it is not even Greek. But it, too, has an aura, an aura that was formed by centuries of praise. Other statues here are comparably familiar. The mental images we carry of Sophocles and Pericles and the Emperor Augustus are based on antique marbles included in the show. Christians may not views these things as sacred, but they are relics still.

Other objects here exact another sort of reverence. The "St. Jerome" on view, though damaged and unfinished, was painted by Leonardo and therefore carries with it the nimbus of his fame. We know that Michelangelo, who designed St. Peter's dome, admired that Greek torso; and that the great Bernini, who conceived St. Peter's colonnade and its swirling Baldacchino, worked those terra cottas. Sassetta, Fra Angelico, Caravaggio and Matisse are represented, too.

The Vatican exhibit is more diffuse than expected. It does not stay in focus. Its 237 objects evoke too many thoughts and call too many ghosts--ghosts of Ben Shahn and Napoleon, of pharaohs, popes and pirates, of long-dead Roman lovers, and of shivering scholar-priests gathering bark idols in Tierra del Fuego. A number of the greatest artists of the church--Michelangelo, Sangallo, Bramante--are represented here only by reflection. The handsome installation is not chronological. Objects secular and sacred, commissioned and acquired, stand here side by side.

The Vatican exhibit shimmers in the memory. It loses as it does so the concentrated impact of such blockbuster exhibits as the Tutankhamen show and the Picasso retrospective. It could not be more varied. The monument that's missing, the Vatican itself, which looms above these objects, lends this exhibition what unity it has.

That city-state was built where the historical St. Peter met his martyrdom in Rome sometime between 64 and 67 A.D. He was crucified, upside down, in the Circus of Nero. His body was then placed in a necropolis nearby. Although he was interred in haste, his tomb was not forgotten. The church called Old St. Peter's, the wood-roofed basilica built by the Emperor Constantine between 320 and 330 A.D., was constructed on that holy spot. So, too, was the huge church that replaced it at the height of the Renaissance. The high altar of St. Peter's, the greatest church in Christendom, stands just above his grave.

An imagined line of light--the holy lineage that runs from Christ to Simon Peter, and from Peter to Pope John Paul II--flickers in this show.

In a number of its objects that lineage is manifest. An old and much-restored fragment of mosaic comes from the eastern inner wall of Old St. Peter's. It is a portrait of Pope John VII, who reigned from 705 to 707. The square halo that surrounds his head signifies that he was still alive when portrayed. Nearby are displayed six applique' reliefs of Christ and five apostles. These, too, come from the old church, and were probably installed at the niche of the Confessio--where newly consecrated bishops used to spend the night in prayer. Other objects here--the altar candlesticks and crosses, the medieval reliquaries, and the vestments of Pope Clement VII--also shine with holiness.

But the popes have long collected with surprising liberality and for reasons less explicable--for scholarship, or pomp, or mere curiosity. The Vatican now boasts 10 separate museums, and not all of these, by any means, house works of Christian art.

This exhibit does not proselytize. Many of its most famous and impressive things suggest Christ not at all.

The Apollo Belvedere, the Belvedere Torso, the Egyptian granite lions, Roman portrait busts and grand Etruscan bronzes tell us vastly more about the pleasures of the pontiffs than they do about their piety. It is possible to argue that the beauty of Apollo is tied to that of Christ, that statues of Egyptian gods are relics of the Holy Land, and that the grandeur of old Greece and Rome support that of the church. But such claims have a hollow ring. A sort of Catholic thought-veneer has been given to these ancient things by the Vatican that houses them and by the popes who treasured them. But still a pious Christian might regard much of this show as pagan to the core.

Many of its objects rightfully belong today to history, to scholarship, to all of Western thought, not only to the church.

Consider, for example, the Apollo Belvedere, its centuries of glory and its later fall. No other statue tells us more about the changing ways in which the West has viewed the highest realms of art. It was installed in the Vatican between 1503 and 1509 by Pope Julius II, who regarded it as both beautiful and Greek. Most scholars nowadays would judge him wrong on both counts. It isn't Greek, but Greco-Roman. It is almost certainly a copy of an antique bronze original, one of many such, made in the second century to decorate the gardens of the Roman rich. Once it was the most admired marble in the world, yet now it's often sneered at. Recent writers have described its great fame as "ridiculous" and the work itself as "little short of abominable." To modern eyes, writes Joseph Alsop, "the statue appears disturbingly mechanical and pseudo-grandiose."

Yet men with eyes as sharp as ours once saw in the Apollo beauty unsurpassed. It was taken as a model by Raphael, Dur er, Bernini, Michelangelo, and countless lesser artists. It swept Go ethe off his feet. "It is," wrote J.J. Winckelmann, that hugely influential critic, "the highest ideal of art among all the works of antiquity. Enter, O reader, with your spirit into this kingdom of beauty incarnate . . . "

"Unfortunately for the modern reader," writes Kenneth Clark, "this kingdom is closed . . . In no other famous work of art, perhaps, are idea and execution more distressingly divorced."

The Apollo Belvedere, like many other objects in the Vatican exhibit, has just been restored. (The Vatican was given $580,000 for such restorations by the three American museums that will exhibit the show.) Its phony hands are gone; so, too, is its fig leaf. But the slender god still wears a coat of history that cannot be removed.

The same is true of many other objects in this show. It is now nearly impossible to view the mutilations and the knotted muscles of the Belvedere Torso, an authentic Greek original, without thinking of its impact on the art of Michelangelo. Nor can Donato Credi's "Astronomical Observations" of 1711, which prompted the construction of a papal observatory, be seen without recalling Galileo's earlier wrestle with the church.

There are tapestries on view here that Raphael designed for the Sistine Chapel. When they were still new they were "universally believed . . . the most beautiful things in the world." What has happened to them since makes them even more impressive. They were taken from the Vatican in 1527 during the Sack of Rome. Pirates stole them later and took them to Tunisia. They went from there to Venice, and to Constantinople, where at last they were purchased for return to the church.

The Vatican exhibit, now being previewed in Manhattan, will be on public view there until June 12. Tickets are required; many are available through Ticketron ($4.80) and Teletron ($5.50). The show will then go to Chicago, and to San Francisco. A number of its objects--the small painting by Sassetta, the 15th-century frescoes of sweet music-making angels, and the 1604 Caravaggio "Deposition"--are beautiful enough to stop one in one's tracks. But the exhibit as a whole, with all its layered histories and accumulated auras, somehow excites the awe as much as it does the eye.