Some people are shocked to learn about the table manners of Christ and the apostles. Like decadent Romans, horrible dictu, they dined in the horizontal position.

Orientals, of course, consider it bizarre, and perhaps a little vulgar, that we pitch food into our mouths with metal forks. "(Cannibals, incidentally, use forks only for cannibalism, considering animal meat fit to eat with their hands.)

Unisex hot tubs were quite the rage in medieval Europe -- a far naughtier period than Victorian historians have led us to believe. According to a Flemish painting, at least one lusty wedding banquet was held in hot water, the food-laden boards set across a row of tubs. Now there is one for Californian play people to emulate!

Voltaire read his "Pucelle" to Madame du Chatelet in her bathroom, a place which in other periods and cultures often enjoyed far greater sybaritic prestige -- and more amorous and murderous (Agamemnon and Marat!) passions -- than it does today.

These, and many more intriguing behavorial tidbits, picked with obvious relish by Bernard Rudofsky from the world's raisin cake of cultural history, are the subject of an exhibition that opened last week in New York City at the Cooper-Hewitt, the Smithsonian's National Museum of Design. It continues through Feb. 22, 1981.

The redoubtable Rudofsky is an Austrian-born architect, engineer, historian, critic, writer, and teacher -- one of those rare generalists, in short, who can still see the forest of our society for its over-specialized trees. He manages to say important things -- with Schlagoberts, a whipped blend of peculiarly Austrian wit, irony, eruditon and charm.

Some years ago he gave us a show and a book on "Architecture Without Architects," revealing what everybody always knew but nobody dared to say -- namely that folk architecture, as on Mediterranean islands or in African villages, is inevitably more attractive and functional than housing designed by modern architects. The show was held at New York's Museum of Modern Art. He has written several other books since.

This show is much like a good conversation on exhibit. It does not attempt to cover the subject, but dabbles here and there. It doesn't have much structure, but arrests, entertains, stimulates and even instructs.

The title of the show as well as the book (Doubleday Anchor, $10.95) is "Now I Lay Me Down To Eat." The subtitle is "A Salute to the Unknown Art of Living," which is undoubtedly the highest art of them all.

As the subtitle implies, Rudofsky feels we Occidentals are not very good at it and shows us why in words, art works, book illustrations, artifacts, utensils and a lot of photographs of all of the above. The materials covers customs and manners as recorded and distorted at various times and places, including Rudofsky's comments. With much glee he observes time and again that people did not always behave the way we are told they behaved.

In the case of the apostles, who play the title role of the show, as it were, this seems irrelevant. There is no doubt that they ate the Last Supper in a reclining position. So did all upper- and middle-class Romans and Jews in their time. It must have benefited their digestion, as sedentary eating tends to cramp sensitive intestines. But then, they allowed no women at the table, which must have cramped their conversation.

Rudofsky introduces any number of ancient murals and miniatures with apostles on the couch to prove his uncontested case. In fairness, he might also have told us that the painters of The Last Supper never aspired to scientific accuracy. Leonardo, after all, was not illustrating to The National Geographic.

If accurate cultural anthropology is to be our criterion of art, what, pray tell, are snow, mountains, medieval towns and Alpine stables doing in the Christian art of nearly 20 centuries?

Christ's table manners detract somewhat from the exhibition's central and fascinating theme: the reciprocal relation between behavior and implement, skill and tool, which is the basis of civilization and shapes the human environment.

Rudofsky explores eating and eating utensils; sitting and seats; sleeping and beds; bathing and bathrooms, tubs, toilets, bidets and the like. He has a great deal of fun twitting us about the moral -- and morality -- of it all. It's not just that the apostles were eating and drinking in a recumbent position. It is also that at various times in history perfectly respectable people enjoyed cozying up, six or more in one bed -- or one tub; that an 18th-century painting by Louis Leopold Boilly shows a lady using her bidet; and that throughout history people often openly acted as lustily and hedonistically as Hugh Hefner would have us act.

This is a good show to see as the Moral Majority seems to gain influence -- as we seem to believe, in Rudofsky's words, "that an acquaintance with alien life styles jeopardizes one's national identity."

The point is not to approve or disapprove of lying, sitting or standing as we eat. Nor is it that we seem excessively immoral. Our trouble, it seems to me, is that our mess culture is excessively vulgar, which is a lot worse.

We are also excessively intolerant. "Our dangers," Rudofsky quotes Judge Learned Hand, "are not from the outrageous but from the conforming: not from those who rarely and under the lurid glare of obloquy upset our moral complaisance, or shock us with unaccustomed conduct, but from those, the mass of us, who take their virtues and their tastes, like their shirts and their furniture, from the limited patterns which the market offers."

The Cooper-Hewitt and its director, Lisa Taylor, have shown us different patterns and they have done so in a new, a somewhat personal, rather literary but literate genre of exhibition. It is to be encouraged.