A terrible duty has become a terrible beauty. What a difference a generation makes. Ingmar Bergman is on at the National Film Theatre for two months, in a complete retrospective, and suddenly we experience the shock of the old made new. The films are the same but they seem re-polished by the privilege of choice. Moviemanes can volunteer to see works they were once peer-pressured into by arthouse conscience, hairshirt cinephilia or student-era duty. (Come on, own up: you know it's true if you were alive and filmgoing back then). And just as ex-schoolchildren will start reading and enjoying classics when they no longer have to, when homework becomes recreation and auto-didacticism replaces ought-to didacticism, Bergman may be entering the sublime afterlife of post-fashionability. It is one "post" we can welcome in an epoch that, God knows, has been post-this, post-that, post-everything.

We know that at a certain hour of a certain year–no one has yet pinned it down–modernism disappeared. Everyone was swept up in the Postmodernist Ball: "Come dressed as something pre-Bauhaus." European art cinema, in the stern old sense of monochrome movies about human emotional or spiritual agony, became yesterday's masochism. I reckon it happened in the late 1960s, when Bonnie and Clyde and Easy Rider burst onto screens. Suddenly America, not Europe, was making brave new cinema. It was also combining art with entertainment, thereby proving, after years of art-equals-difficulty (Godard, Antonioni, Pasolini, and yes, Bergman) that cineastes could have their cake and eat it. Altman, Scorsese, De Palma and Malick flocked into the kitchen, made more cakes, and announced loudly that art cinema did not mean intellectual overload or sensory deprivation. By the time the movie brats arrived–Spielberg and Lucas–we assumed they were part of the same master-baking team and nodded in their Toytown recipes as if they were high art.

Some of these men were, and are, big filmmakers. Of others one could ask, as originality has given way to nostalgia, pasticherie and Tinseltown intertextuality (aka goofing off about films you love), whether they are master-bakers or just something that rhymes with it. For every Tarantino who transmutes film buffery into inventiveness there are Zemeckises, Lucases, Dantes and Carpenters who diddle around with old B-movie, film noir or adventure yarn styles and turn cinema into a recyclist's toyroom crammed to the ceiling with bric-a-brac.

Bergman's cinema seems so uncluttered, even when set next to Europe's present-day auteurs, that on re-seeing Wild Strawberries, Winter Light or Cries and Whispers one wonders what happened not to difficulty but to simplicity. It is like hearing the solo instrument come back after a long orchestral section in a concerto. Nothing wrong with the symphonic bits, but how uplifting that pure single sound is. Bergman's films are about people going naked before each other. Not physically, though sometimes that, but morally, emotionally, psychologically. Bergman's great stories are cathartic tragedies of honesty. He took the best from two Scandinavian drama kings–Ibsen and his theme of the unrevealable that must be revealed, Strindberg and his pre-Sartrian "hell is other people"–and modernized them for a post-Freudian age of confessional candour, of the clinical baring of personal breakdown or domestic breakup. Bergman's films are mercilessly truthful. At the same time, like all great artists, he knows that truth is more than realism. Realism is the bottom rung of the ladder. If your art does not move on to some kind of transcendence–in Bergman's case a gymnastic neo-gothic expressionism that could move freely between the naturalistic and the grotesque–you are just a commentator not a creator.

The National Film Theatre season shows, fascinatingly, how that transcendence began. Watching Bergman's earliest, stage-play-based films–It Rains on Our Love (1946) or A Ship Bound for India (1947)–you can catch the moments when a competent journeyman mise-en-scène done with note-perfect actors rises to something more truthful because it is more hyperrealistic. In the second film there is a long, vertiginous overhead shot of a salvage captain and his semi-estranged wife, who as they talk from bunk to bunk in a darkened cabin sluiced with eerie water-light–she above, he below–begin to strip-mine the memories of a ruined marriage. It is a stunning scene, looking straight into mature Bergman across a gap of 10 years.

In later films–his greatest for me are The Silence, Hour of the Wolf and Persona–the hyperreal took over the whole show (while never smothering the truthful). In these movies Bergman's characters wander around inside each other's brains. In the wonderful Hour of the Wolf, the Bergman I picked recently for my all-time cinematic top 10, an artist (Max von Sydow) spends the film's duration trespassing inside other people's minds and souls, while they trespass inside his. Human life is presented as a kind of nightmarish multiple identity quest. (And some people have very impure motives for wanting to get inside your soul). The holocaustic black-and-white visuals suggest some early horror classic, Murnau's Nosferatu or Dreyer's Vampyr, while the true horrors, the film suggests, are inside us not outside. Meanwhile, God is dead, or at best a heartless eavesdropper on sufferings for which He has no balm or answer.

Bergman's heyday, coinciding with Godard's, Bunuel's, Fellini's and Antonioni's, was European cinema's finest hour. No one would wish away the films of Almodovar or Ozon. But they carry glam-rock postmodernism in their bloodstreams. If you took away the allusions to other movies in 8 Women or All About My Mother you might just start unpicking the whole work. Bergman had influences too: not just Ibsen and Strindberg but Nordic movie forebears Sjöström and Stiller and even his beloved Fellini. But the influences aren't worn like designer labels. They are there to nourish his vision, then disappear into it. His cinema is organic in the best sense. Everything serves the beating heart of a passionate, questing, intelligent perspective on human nature. Bergman's films aren't funny and never were. But it is amazing how exciting, fierce and primal they have become now that–let's say it again–duty has turned into beauty.