Venice: Nothing can prepare tourists for the wonder of St Mark's Basilica

In the dark, a church interior can be an intimidating place. The uncertain shuffling of the tour group echoes as if a flight of bats is flapping around the dome and, when someone coughs, it sounds like the bark of the devil. Beneath us lies a crypt full of bones, and saints stand bleeding in the gloom. We feel our way onto wooden chairs. Then suddenly, with a great cluck-clunk-clunk of successive switches, the lights spring on and a luminous gold heaven is revealed.

There's scarcely an "ooh" from my fellow cruise passengers, just a shocked and bedazzled silence as St Mark's Basilica is revealed in its golden glory. Biblical figures spring from the darkness in a blaze: saints and seraphim, prophets and angels. My neck cricks as I gaze at the vast Christ Pantocrator that dominates the central apse. Every golden glint on his tunic, every fold in his blue robe is lovingly detailed. He faces the length of the cathedral, one arm upraised in blessing, fingers long and tapering. An expression of compassion transforms his gaunt face; soulful heavy-lidded eyes seem to look right at me. I stare back and marvel.

Some 8000 square metres of other mosaics shimmer; individual glass tiles are fused to a backing of gold leaf, designed to resemble the craftsmen's notion of divine light. Surrounded by aquamarine, green and gold, I feel as if I've stepped into an illuminated manuscript, upsized and turned into three dimensions for the world's wonderment. It's a magical moment in this building a millennium old and saturated with history.

We started our Uniworld shore excursion, on our final cruise evening in Venice, feeling like burglars. A lone guard unlocked a side door to the basilica and we crept inside to the puzzled glances of tourists passing across the square. During the daytime, we'd have been queuing to get in. Now we're alone but for a small German tour group. Uniworld bills the visit as exclusive, but it isn't quite; St Marks opens most nights, though you have to be part of a tour group for the privilege. The advantage is the unrivalled evening atmosphere and access to otherwise closed parts of the basilica. We're also lucky to have informative commentary from our Uniworld guide Susan Steer.

In the porch, mosaics tell stories from the Old Testament, and relate the tale of St Mark's body, stolen from the Middle East and smuggled to the city in a pork barrel in 1270, thereby fulfilling a convenient prophecy that St Mark's bones would be laid to rest in Venice. The basilica had already been built by then – it's largely an 11th-century building – but required a saint's cachet. The mosaics here are Bible turned into comic strip, albeit a rather refined one. Eve is tempted by an apple, Noah builds his ark, Abraham is sacrificed.

In the interior, large-scale figures from the New Testament take over: the Virgin Mary and apostles, the life of St Mark, a bearded congregation of prophets. In the dome, blood from a mosaic dove dribbles onto a circle of saints. The decoration is more reminiscent of Orthodox than Catholic churches, with a Greek cross layout, Byzantine-style dome and walls slabbed in marble from the Eastern Mediterranean. "The whole thing basically floats on a raft of massive tree trunks sunk into the marshy lagoon," says Susan.

Having toured the body of the basilica, the guard folds back the Pala d'Oro, a magnificent gold altarpiece commissioned by a doge in 976, studded with hundreds of gemstones, where enamel saints stand vigil over the sarcophagus of St Mark. Then we descend into the crypt, off limits to the general run of tourists. It feels damp and musty; for a thousand years it has been subject to Venice's high tides, which have also caused undulations in the basilica's sumptuous tiled floors.