Modernism is regarded by its sometimes fanatical adherents as being very much more than a matter of style. Indeed, to the true believer Modernism (always with that upper-case 'M') does away altogether with such antediluvian notions as style in favour of super-rationalism and pared down simplicity. Small wonder, then, that Modernism was seen as hostile to the Church. As early as 1907 Pope Pius X issued a Vatican Encyclical condemning Modernism as heretical because it encouraged atheism and "the annihilation of all religion". He was right to be alarmed. When you start from scratch, with the idea of creating a better and more rational world, you are inclined to be progressive, with little love for trappings of the past such as religion. And while there is a Modernist moment in all the creative arts, from literature and painting to furniture design and music, it is in architecture where the movement is most firmly fixed in tangible notions of social change, of novel approaches to how to live – forefronting function over ornament and reason over faith. There was an assumption that buildings could transform lives, relationships, traditional formations of family and community, our attitudes to the environment – and in doing so would sweep away obsolete beliefs and replace them with a new vision. In the end however, Modernism did not go to war with the church, nor become the clean and irrevocable break with the past it wanted to be. Instead it lost faith in its own lack of faith, made friends with the Church, and became, in the end, just another, if hugely influential, style.

Modernism should not be confused with 'modern'. Modern means anything that is happening now, whereas Modernism connotes a specific period, and philosophy, within architecture long since past. It is quite possible therefore to have a modern movement in architecture which is explicitly anti-Modernist, as happened during the highpoint of postmodernism in the mid-1980s. When architect Terry Farrell placed big colourful eggcups on the roof of his TV Am building – which functioned both to reference more traditional forms of ornamentation and, in the allegedly witty way postmodernism does, comment on what was taking place within – the gesture was very modern. But it wasn't Modernist (except for diehards, who pointed out that postmodernism was really just big, ordinary late-Modernism in fancy clothes).

There is plenty of controversy over when the Modernist period started and ended and any decision, such as that of the Victoria and Albert museum to concentrate their major exhibition 'Designing a new world' on the period 1914 to 1939, is likely to be somewhat arbitrary. Arguably Modernism begins well before that. The 19th century had been a rag-bag of historical styles: industrialisation, and its attendant upheavals, had had the effect of making people constantly ransack and rearrange the past. Rapid industrial and technical advance was traumatic, which is why so much of the art of the period, from gothic revival church architecture to the paintings of the Pre-Raphaelites, represented a rejection of modernity. Mind you, the Victorians could always crank out something stupendously advanced, such as the Crystal Palace (later much admired by Modernist pioneer Le Corbusier), when the need arose.

In contrast, Modernism was all about scraping away the traces of history and the styles of bygone eras, eliminating in particular anything fussy or ornamental. Modernism is generally (though not universally) associated with a desire to make a clean break with the past. This was most true in Europe – which in the aftermath of the World War I had every reason for wanting a new beginning. Many war memorials were built with little or no religious imagery, suggesting a loss of belief in the securities of tradition. It was Edwin Lutyens – no Modernist he – who devised the idea of the Cenotaph, or empty tomb, as a truer act of remembrance than an appeal to a higher power.

The Modernist credo was to reject the pretences of style and seek to discover instead the universal verities of form and function and record them in the materials of modernity – steel, glass, concrete. This is why so many disliked the curatorial phrase 'International Style', coined by Henry Russell Hitchcock in 1932 for an exhibition at the new Museum of Modern Art, which has, however, stuck ever since. Serious adherents preferred 'Modern Movement'.

In 1919 Walter Gropius, the most influential director of the Bauhaus school of design in Dessau, which was the incubator of Modernism, summed up this radical dissociation from style: "Today's artist lives in an era of dissolution without guidance. He stands alone. The old forms are in ruins, the benumbed world is shaken up, the old human spirit is invalidated and in flux towards a new form. We float in space and cannot perceive the new order."

Modernism was partly a dream of industrialised building, partly a desire to synthesise all the arts. It is often forgotten that the Bauhaus (key teachers being Gropius, Mies van der Rohe and Hannes Meyer) was as much about avant-garde theatre productions and bent-steel chairs as it was about concrete apartments and glass skyscrapers. Meanwhile in France, Le Corbusier – the leader of a more Mediterranean Modernism – adored aeroplanes, cars and ocean liners, and compared them to the Parthenon in what he saw as their purity of construction.

He was also an artist and designed furniture. This multi-tasking was commonplace: even philosopher Ludwig Wittgenstein had a short early career as a Modernist architect. Above all it was a matter of a rational response to contemporary needs, using the most advanced techniques and materials available.

Housing was a particular concern of Modernists: they believed in the health and social benefits of clearing and rebuilding entire cities on rational lines and this attitude persisted right through to the 1970s. And of course it did become a very distinct style by the 1930s: white-walled, flat-roofed, strip-windowed structures, often erupting into elements of curving, steel-framed glazing.

The period 1914-1939 marks the ascendance of Modernism, its 'heroic' age. A time of feverish activity and building in Germany and across Europe, when the Bauhaus intellectuals were drawing, designing, writing and arguing. With its radicalism and lack of respect for tradition, Modernism inevitably ran afoul of the Nazis. The Bauhaus school, very much associated with the Weimer Republic, was closed by the Nazis in 1933, and the Bauhaus members, many of them Jewish, dispersed throughout the world. Modernism's most prominent representatives – Gropius, Mies Van De Rohe and Moholy-Nagy – ended up in America. Modernism, the great anti-style, committed to radical social change, became the most recognisable style of postwar corporate America.

America had a domestic Modernism, but in contrast to the austere, geometric Europeans, American Modernists such as Frank Lloyd Wright and composer George Gershwin were evolutionists rather than revolutionists. Although Wright apparently reached his Modernist zenith with the white spiral of New York's 1959 Guggenheim Museum, that building still contains vestigial ornamental aspects of a kind that the diehard modernist, remembering Adolf Loos's 1908 dictum 'ornament is crime', must have eyed askance.

Postwar, Modernism did not so much die as break into many different sub-movements. There is, of course, never a clean break between any one such cultural movement and the next, though it is always fun to try to concoct one. Which is what British-based American academic Charles Jencks famously did when he asserted that Modernism was dead – and this had happened in a certain place at a certain time. The date was 1972, the place St Louis Missouri, the event the dynamiting of the Pruitt-Igoe housing project, a failed American social housing scheme of the 1950s, designed by architect Minoru Yamasaki, more famous as the creator of the twin towers of the World Trade Centre. The demolition of this ambitious but deeply unpopular monument to the rational social-housing dream of the Modernists, in Jencks' view, spelled the end of Modernist self-belief. In Britain we might date architecture's period of doubt and reappraisal from the earlier part-collapse of the Ronan Point tower block in East London in 1968. But what happened between 1939 and then?

Even Jencks acknowledged that architects had been rejecting mainstream Modernism for some time. Robert Venturi, for instance, the American architect of the National Gallery's Sainsbury Wing, had been designing buildings with a postmodern ethos as long ago as the early 1960s. Then there was the architectural power grouping known as Team X, which in 1956 stormed the Modernist ideological citadel of CIAM (Congrès Internationaux d'Architecture Moderne) originally set up in 1928 by a group which included Le Corbusier. Team X was a collection of young radicals charged by CIAM – by then little more than a Le Corbusier fan club – with the task of organising the 1956 congress. Their response 2 to crudely paraphrase – was to effectively destroy the idea of an anonymous 'International Style'.

In fact, in his later, highly sculptural work Corbusier himself had swung well away from this troubled concept. The preceding year his extraordinary, organic, uncategorisable pilgrimage chapel in Ronchamp, north-east France, had been dedicated. No way was that ever International Style. It was and is unique, and uniquely influential. And it was not built from a position of faith – unlike the contemporaneous Coventry Cathedral. That design had come to its architect Basil Spence, he claimed, as a heavenly vision while he was under gas in the dentist's chair. Spence was in the monumental romantic tradition. Le Corbusier was not. When he accepted the commission for the chapel from the local bishop in 1950, he made his impeccably Modernist position clear. "I don't care about your church, I didn't ask you to do it. And, if I do it, I'lll do it my way. It interests me because it's a plastic work. It's difficult."

This apparent paradox – a rejection of the monolithic and austere Modernist aesthetic but a continued adherence to defiantly anti-mystical Modernist practice – inspired Team X whose members and acolytes, such as Aldo van Eyck, Peter and Alison Smithson and James Stirling, went on to become some of the 'signature' architects of their day.

All this mightily troubled the great architectural historian Sir Nikolaus Pevsner. He discussed what for him was an unwelcome development in a 1966 BBC Third Programme talk, 'The Anti-Pioneers'. There was a new spirit abroad, said Pevsner: "… a new style, successor to my International Modern of the 1930s, a postmodern style I would be tempted to call it, but the legitimate style of the 1950s and 1960s." Pevsner, speaking a decade before Jencks, thus regarded personality architecture such as Stirling's as postmodern, in effect a new outbreak of Expressionism.

Note that Pevsner used the loose 'modern' rather than more specific 'Modernist'. That is because, for him as for so many others, Modernism had been the end of history so far as architecture was concerned, and so therefore was destined to be perpetually fresh, always modern. Surely nothing could replace it? However, he accepted with good grace, even humour, that it had indeed been replaced. He had called his talk 'The Anti-Pioneers', in reference to the fact that it was 30 years since he had published one of Modernism's key texts, Pioneers of modern design, in which he had set out to show the great lead-in to architecture's glorious final chapter, from William Morris in the mid-19th century to Walter Gropius in 1914. 'Simplicity, honesty, service' were the key Modern Movement ingredients for Pevsner.

"What I thought I described was the coming of the Millennium," confided Pevsner to his Third Programme audience. "To me what had been achieved in 1914 was the style of the century. It never occurred to me to look beyond. Here was the one and only style which fitted all those aspects which mattered, aspects of economics and sociology, of materials and function. It seems folly to think that anybody would wish to abandon it. But human feelings are inscrutable and what we are experiencing now is a new style completely, an anti-Pioneers style… alarmingly harking back in many different, even contradictory ways to Art Nouveau and to Expressionism."

To Pevsner, to Russell Hitchcock, to CIAM, to Mondrian, to Schoenberg, Modernism was meant to be the culmination of everything. This is why, when discussing it, almost messianic religious phraseology is often deployed: odd, when you consider the essentially secular nature of the movement.

The true believers did not go away: they were merely eclipsed for a while, and then shone again in a new generation. But having been successfully challenged twice since the World War II, it makes no difference that architectural Modernism had returned with a vengeance by the mid 1990s, kicking phase two postModernism into touch. That just meant the start of phase three.

We are all postmodernists now. As our artists look to Dada, not Picasso and Matisse, so all our globe-trotting 'signature' architects are revival Expressionists as Pevsner foresaw: Frank Gehry, Zaha Hadid, Rem Koolhaas, Daniel Libeskind, Herzog and de Meuron. The new international style is the now familiar expressionism of swooping curves and jagged angles, the blurring of the distinction between levels. But does that mean that the old, upper-case International Style has been relegated to the history books? Not exactly. For a start, many other architects have gone back to the motherlode to build in the tradition of the original International Style. 'Simplicity, honesty, service' remains the watchwords of many others. All of these twists, departures and returns are simply varieties of revivalism. Modernism is old enough to have developed its own complex history, which has become a resource to draw on like any other. But in architecture, it stubbornly remains more than that. It's still a manifesto for a better, healthier, fairer, more rational world.

Hugh Pearman is architecture and design critic for the Sunday Times. 'Modernism: Designing a new world' is at the V&A April 6 - July 23. 'From the Bauhaus to the new world' is at Tate Modern March 9 - June 4