ARCHITECTURE: Around the City's pretentious monuments to Mammon hangs a necklace of divinity

Last Saturday I had the privilege of conducting the Friends of Fitzwilliam Museum, Cambridge, on a tour of City of London churches. Those 40 or so churches in the Square Mile that have survived fires, bombs and rapacious commercial development are often locked or, if they are open, their hours are as byzantine as those of their counterparts in Venice. While it would be nice to think that they could be open more often than they are, their secretiveness makes us only the more curious to see inside them.

And what treasures they reveal. Sir Christopher Wren rebuilt 51 of the 97 churches that stood within the City walls before the Great Fire of 1666; 23 of his churches survive. Together with St Mary Woolnoth, an astonishingly powerful design by his one-time assistant, Nicholas Hawksmoor, they form an architectural necklace of quiet, yet abiding and inspiring beauty around the neck of an urban quarter devoted to the ways of Mammon. The survival of each is a reminder that money (pace Baroness Thatcher, Nigel Lawson, etc) is far from being all that counts, or can be counted. The City might have changed out of all recognition during the champagne-fuelled boom of the mid-Eighties, yet St Mary Abchurch or St Margaret Lothbury remain far finer, more endearing and more enduring buildings than all money has bought in terms of air-conditioned, Post-Modern dealing rooms and trading floors.

The City churches are, however, more special than this. Those by Wren and Hawksmoor are, almost without exception, minor masterpieces, expressions of intelligent, questioning and exquisite architecture squeezed into tiny squares and behind the monuments to Mammon. They remind us that a coach ride from Cambridge to the City of London is sufficient to lift us into the realm of overlooked architectural genius. Too often today we seek out the exotic in a perpetual quest to find buildings or aesthetic and spiritual experiences more beautiful, more inspiring and less tainted than those we have at home.

Push open the doors of St Mary Abchurch, and there is all the spatial and spiritual beauty you ever really need. A simple red brick and Portland stone box turns out to be a near-perfect architectural jewel-box inside. A painted dome rises weightlessly from white walls. Shafts of light explore its soft curves. Here is an altarpiece decorated sparingly, yet to poetic effect, with fruit carved by Grinling Gibbons. It would have had John Keats, a City of London lad, reaching for his notebook. And as if to show how truly civilised Wren made these City churches, that corner over there was where the dog-kennels - pews for canine worshippers - used to be ("All things bright and beautiful ...").

As we walked from church to church, our Cambridge gang saw how in this lovely litany of Reformation churches (a foil to the Chapel of the Holy Shroud shown opposite this column), gifted architects experimented with plans and sections, so that within a single square mile we have a taste of many of the arcane theories of Classical architecture and of many of its great practitioners. Here we find Wren experimenting with themes adapted from Renaissance Italy, from the Protestant Netherlands or from ambitious France. Here, too, in the shadow of the Bank of England, the Royal Exchange and the Mansion House, we find Hawksmoor reinterpreting Borromini's wilful and decidedly Catholic Baroque into an architecture at once powerful, Protestant and, despite its cleverness, as English as an eighteenth-century yeoman farmer.

Next week, the Friends of the City Churches have arranged for all of these churches (together with the Spanish and Portuguese Synagogue at Beavis Marks - a "must") to be open on two consecutive days.

It is an experience not to be missed. You have nothing to lose but the soles of your shoes, while the discovery of so much architectural splendour and ingenuity so close to so many embarrassing and pretentious buildings of the money-mad Eighties is close to divine revelationn

City Church Walk, 24 and 25 April. Registration from 10am, St Mary Aldermary, Bow Lane, London EC4; pounds 5 one day, pounds 8 both days. For further information call Friends of City Churches on 0171-228 3336