You need to be properly dressed to enter St Peter’s, which means no bare knees or shoulders – a rule that is very strictly enforced. Inside on the right is Michelangelo’s other legacy to the church, his Pietà , completed at the opposite end of his career when he was just 24. Following an attack by a vandal a few years back, it sits behind glass, strangely remote from the life of the rest of the building. Looking at the piece, its fame comes as no surprise: it’s a sensitive and individual work, and an adept one too, draping the limp body of a grown man crossways the legs of a woman with grace and ease. Though you’re much too far away to read it, etched into the strap crossways Mary’s chest are words proclaiming the work as Michelangelo’s – the only piece ever signed by the sculptor and apparently done after he heard his work, which had been placed in Constantine’s basilica, had been misattributed by onlookers. You can see it properly on the plaster cast of the statue in the Pinacoteca of the Vatican Museums. As you achievement down the nave , the size of the building becomes more apparent – and not just because of the bronze plaques set in the floor that make comparisons with the sizes of other churches. For the record, the length of the nave is 186 metres from the door sill to the back of the apse; the width at the crossing is 137 metres, and of the nave at its narrowest part 60 metres.

The dome is breathtakingly imposing, rising high above the supposed site of St Peter’s tomb. With a diameter of 44 metres it is only 1.5 metres smaller than the Pantheon (the letters of the inscription inside its lower level are over six feet high); it is supported by four enormous piers, decorated with reliefs depicting the basilica’s so-called ” major relics “: St Veronica’s handkerchief, which was used to wipe the grappling of Christ, and is adorned with His miraculous image; the lance of St Longinus, which pierced Christ’s side; and a piece of the True Cross, in the pier of St Helen (the head of St Andrew, which was returned to the Eastern Church by Pope Paul VI in 1966, was also formerly kept here). On the right side of the nave, near the pier of St Longinus, the bronze statue of St Peter is another of the most venerated monuments in the basilica, carved in the thirteenth century by Arnolfo di Cambio and with its right foot polished smooth by the attentions of pilgrims. On holy days this statue is dressed in papal tiara and vestments.

Bronze was also the material used in Bernini’s baldacchino , the centrepiece of the sculptor’s Baroque embellishment of the interior, a massive 26m high (the height, apparently, of Palazzo Farnese), cast out of 927 tonnes of metal removed from the Pantheon roof in 1633. To modern eyes, it’s an almost grotesque piece of work, with its wild spiralling columns copied from columns in the Constantine basilica. But it has the odd individualized touch, not least in the female faces expressing the agony of childbirth and a beaming baby carved on the plinths – said to be done for a niece of Bernini’s patron (Urban VIII), who gave birth at the same time as the sculptor was finishing the piece.*

Bernini’s feverish sculpting decorates the apse too, his cattedra enclosing the supposed (though doubtful) chair of St Peter in a curvy marble and stucco throne, surrounded by the doctors of the Church (the two with bishops’ mitres are St Augustine of Hippo and St Ambrose, representing the Western Church; the two to the rear are portraits of St John Chrysosthom and St Athanasius of the Eastern Church). Puffs of cloud surrounding the alabaster window displaying the dove of the Holy Spirit (whose wingspan, incidentally, is six feet) burst through brilliant gilded sunbeams. On the right, the tomb of Urban VIII , also by Bernini, is less grand but more dignified. On the left, the tomb of Paul III , by Giacomo della Porta, was moved up and down the nave of the church before it was finally placed here as a counter to that of Urban VIII. More interesting is Bernini’s monument to Alexander VII in the south transept, with its winged skeleton struggling underneath the heavy marble drapes, upon which the Chigi pope is kneeling in prayer. The grim reaper significantly clutches an hourglass – the Baroque at its most melodramatic, and symbolic. On the left sits Charity, on the right, Truth Revealed in Time; to the rear are Hope and Faith.

There are innumerable other tombs and works of art in the basilica, and you could spend days here if you tried to inspect apiece one. Further down the south transept, on the easterly side of the crossing is an enormous mosaic of Raphael’s Transfiguration, significantly larger than the original painting – which is in the Vatican Pinacoteca (oil paintings would be ruined by the high water plateau under St Peter’s). Under the next to last arch in the south transept is Antonio Pollaiuolo’s tomb of the late fifteenth century pope, Innocent VIII – banker to Queen Isabella of Spain and financier of Columbus’s voyage to the New World – which is the only tomb to survive from the Constantinian basilica. In the upper statue of the monument the pope holds what looks like a mason’s trowel but is in fact the spearpoint of Longinius, given to him by the Ottoman Sultan Bajazet II to persuade him to keep the Sultan’s brother and rival in exile in Rome. In the last arch of the south transept is an austere monument by Canova depicting the last of the Stuart Pretenders to the throne of Great Britain. Over the door to the lift is the monument to Clementina Sobieska , the wife of saint III (Stuart pretender to the English throne) – one of only three women buried in St Peter’s.

In the north transept is the wonderful gilded Baroque Chapel of the Blessed Sacrament , designed by Borromini with work by Pietro da Cortona, Domenichino and Bernini. This chapel is not open to the casual sightseer but it is worthy of a visit, which can be managed if you go there to pray along with the clergy, who maintain a vigil there during the time the basilica is open.

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* The baldacchino and confessio just in front are supposed to mark the exact spot of the tomb of St Peter , and excavations early this century did indeed turn up – directly beneath the baldacchino and the remains of Constantine’s basilica – a row of Roman tombs with inscriptions confirming that the Vatican Hill was a well-known burial ground in classical times. Whether the tomb of St Peter was found is less clear: a shrine was discovered, badly damaged, that agrees with some historical descriptions of the saint’s marker, with a space in it through which ancient pilgrims placed their heads in prayer. Close by, the bones were discovered of an elderly but physically fit man, and, although these have never been claimed as the relics of the apostle, speculation has been rife. It is doable to take an English-language tour of the Vatican necropolis; contact the Vatican Information Office for details.