The building was immediately recognised as a masterpiece. Today, the magnitude of Bentley's achievement is clear. He created a religious building which, though clearly rooted in the architectural concerns of the 19th century, has timeless qualities. Generations of modern architects, attuned to Brutalism and Le Corbusier of Chandigarh and the Maisons Jaoul, have delighted in the noble austerity of the interior, grateful (as so many people have been) that Bentley's own decorative intentions were thwarted.
Yet perhaps it is the unique character of the building - that it is hard to place in terms of period and style - which has made it one of the least generally appreciated of all the world's great religious buildings. For all its monumental magnificence, it is often ignored by tourists.
The need for a great Roman Catholic cathedral in London had been recognised since 1850, when Pius IX restored the hierarchy of bishops in England. In 1892, when Herbert Vaughan became Archbishop of Westminster, the putative cathedral was still in limbo. Vaughan's immediate predecessor, Cardinal Manning, had been sceptical, questioning the need for a costly building. "Could I leave 20,000 children without education", he asked, "and drain my friends to pile up stones and bricks?" None the less, in 1884 Manning acquired the site on land close to Victoria Station that had been occupied by the old Middlesex County Prison.
When Vaughan made the decision to build the cathedral, he chose his architect through a competition. He was deluged with entries, but abandoned the competition and appointed Bentley, the leading Catholic architect of the day (who had been converted to the faith in 1862).
Born in Doncaster, Bentley began work as an engineer before finding a place in the office of Henry Clutton, a church architect and Catholic convert. At 23 Bentley established his own practice in London. He was a devotee of the Gothic Revival (finding St Peter's, Rome "absolutely brutal") but, because of his religious affiliations, won little work, and what he did build had to be cheap. Before Westminster Cathedral, only the powerful Church of the Holy Rood (Watford, 1900) was anything like a vehicle for his talent.
Cardinal Vaughan was adamant that the cathedral should not be Gothic. Its proximity to Westminster Abbey, a great monument of "real" medieval Gothic, was a potential source of embarrassment - how could a new building compete? Moreover, Gothic architecture demanded a mass of costly carvings.
The basilican model favoured by Vaughan allowed a great brick building to be completed quickly with ornamentation added later, as funds permitted. Vaughan was equally critical of the liturgical implications of Gothic style. By the end of the 19th century, the Church was reassessing its historic liturgy, purging it of extraneous trimmings and stressing the need for lay involvement. A broad nave with unimpeded views of the altar was in tune with Vaughan's thinking.
Vaughan's instinct was that the cathedral should be, in essence, an Italian basilica. Bentley pushed the formula into Christian Byzantine. The taste for early Christian and Byzantine architecture had emerged strongly in the 1890s, with Lethaby and Swainson's detailed study of the Sancta Sophia in Constantinople published in 1894. This stimulated interest, particularly among Catholics involved in the Westminster project, in a style that was the opposite of the flashy Baroque triumphalism of the new London Oratory church in Brompton, completed in 1896.
Convinced by the Cardinal's reasoning, Bentley set out in 1894 on a four-month study tour, which took him through Italy, but not as far as Constantinople, because of an outbreak of cholera there. Despite failing health, Bentley began work immediately on his return with a small group of assistants. The basic plan of the cathedral - a nave and sanctuary covered by four great domes, flanked by passage aisles and lateral chapels - was established in a matter of weeks, although details changed with daily contact between architect and client.
The cathedral was built on a vast concrete slab found beneath the demolished prison on the site. This was extended and topped with Bentley's towering brick basilica. All bricks were hand-made (Faversham stock brick for the interior - always intended to be covered by marble - with bricks from Bracknell, Fletton and Poole for the exterior). There was no metal reinforcement of any kind. Portland stone was used for external dressing. Inside, Bentley selected 60 different types of marble. The four domes were made of concrete. The overall effect of the exterior was crisp and powerful, although some critics found it a little fussy, which could never have been said of the sublime interior. "The instant impression", wrote Lethaby, "is that of reality, reason and power, serenity and peace."
Bentley's decorative scheme has never been completed. His intentions are best represented by the magnificent baldachin set over the high altar on eight columns of yellow Verona marble and by the lavish Holy Souls chapel on the north side of the nave. Decoration, principally mosaics, was added on a piecemeal basis between 1902 and 1962. During the Fifties and Sixties, the marble facings in the nave were extended seamlessly to gallery height. The mosaics Bentley intended to cover the upper levels of the building and the domes are now unlikely to happen. Indeed some of the mosaics executed elsewhere in the building in recent years provide a strong case for caution. Bentley's daughter, Winefride de L'Hopital, suggested that "a puritan coat of whitewash" would be preferable to inappropriate decoration. Even then, many would argue that the manner in which the upper levels of the cathedral soar into ineffable darkness is something to be cherished.
Although Bentley's masterpiece has never been completed as he intended, neither has it been seriously compromised by later alterations. Indeed his cathedral has proved capable of accommodating the work of other forceful designers, too, most notably Eric Gill, whose marvellous Stations of the Cross were finished in 1918.
The cathedral was mercifully spared from damaging reorganisation after the Second Vatican Council. It survives virtually unscathed. Bentley himself, for various legal reasons, could not be buried in the cathedral (he lies in Mortlake cemetery instead), while his death robbed him of the Royal Gold Medal for Architecture, which was to have been his. The cathedral remains as his monument. It is also the supreme monument of an era of British architecture in which strict historical copying gave way to an inventive view of the past, and when the prospects for a rational, but reverential, modern urban architecture seemed bright. Truly modern, yet equally timeless, Westminster Cathedral is one of the world's great buildings.
This article is extracted from the latest issue of the 'Architects' Journal'. The exhibition 'Westminster Cathedral: 100 years of Art, Architecture & Treasures' runs until 15 October, pounds 4, Westminster Cathedral, Victoria Street, London SW1. There is a series of lectures to coincide with the exhibition; for tickets and information call 0171-344 4444.