Say the name, and the mind boggles: Pope Casanova. Yet if history had taken a different turn, a trainee priest named Giacomo Casanova could well have risen to wear the mantle of the most senior spiritual leader on earth.

That is how history’s greatest libertine first made it to Rome: as a young curate with his sights set on the Holy See. (He was nothing if not ambitious, was Casanova.) Had he stayed the course, what parties there might have been at the Vatican... but the world would lack his History of My Life, detailing everything – and everyone – he did after he abandoned the church.

Today, Rome remains both a very sexy city and a profoundly churchly one: think of Anita Ekberg cavorting in the Trevi fountain, and all that baroque architecture, with its marriage of the sublime and the earthy, its orgiastic saints and its dimpled cherubs.

As a guide to this sexy-sacred Rome, you could do a lot worse than the aspiring priest Giacomo Casanova, the original sinner himself. His tales of Rome are a frothy broth of Catholicism, romance, gossip and coffee – which more or less reflects how things have stayed.

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When the 19-year-old Casanova moved to Rome in 1745, as secretary to the pontiff’s right-hand man, Cardinal Acquaviva, his boss was described as “the only man in Rome more powerful than the Pope”. Acquaviva lived in the Palazzo di Spagna, at the bottom of the Spanish Steps. It was, and still is, quite an address.

Casanova had an apartment on the fourth floor; today, it is the Spanish embassy. For a boy from the backstreets of Venice, it was an exciting step into the purpled corridors of power. Snoop around the area and you can still see traces of what was once a sovereign possession of Spain, outside the jurisdiction of the Roman or papal authorities.

“RCDS” is cut into pavements and walls – Regia Corte di Spagna, marking the border of Spanish diplomatic immunity. This is why Casanova, working for a Spanish cardinal, thought he would be able to get away with less than priestly behaviour. He was wrong.

Immediately, he began frequenting the fashionable coffee houses on the “Strada Condotto”, now Via Condotti, and one of these still exists: Caffe Greco. Rome’s oldest cafe, it maintains much of its 18th-century decor, along with the coffee-and-nicotine patina from 250 years devoted to some favourite Roman pursuits: gossip, theology and looking cool.

Not many young priests seem to hang out at Caffe Greco now – prices have moved on, and there’s more Campari than coffee on offer – but it’s worth braving the crowds of shoppers for an authentic experience of old Rome.

Elsewhere, the spirit of religious pomp that attracted Casanova to a career in Rome survives. On my first weekend there, I happened on three sublime moments in churches. First, the scattering of rose petals from the oculus of the Pantheon. (This happens only at Pentecost.)

Second, a Mass at San Giovanni dei Fiorentina that featured exquisite music on its original baroque organ, with the odd howl from an unexpected part of the congregation – this is one of the Roman churches that still allows dogs at Mass, just as in Casanova’s day. And third, a heart-stoppingly beautiful concert rehearsal of Pergolesi’s Stabat Mater at Santa Maria sopra Minerva.

The city seems to offer these dramatic reminders of the divine in our midst every few paces. As Casanova himself observed, if you are undecided about God, in Rome it makes sense to give in to the idea, at least for a while.

The Minerva district, around the Pantheon, was well known to young Father Casanova, because, on the coach to Rome, he had met a woman from that part of town. This is where it all started unravelling for our aspiring pope. Anna Maria Monti changed his path in life for ever – and Casanova insisted that on this and several subsequent occasions he was less the seducer than the willingly seduced. Some women, apparently, have a thing about clergymen.

This is borne out today by an unusual bestseller for the Vatican – a calendar of hunky young priests, many of whom look more set on Casanova’s career than the one for which they have been photographed.

Casanova and Anna Maria went on a jaunt together to Frascati – a delightful short trip out of Rome. Famous for its crisp white wines, the town boasts heavenly views, and several spectacular villas stand testament to the immense power of the church in Casanova’s time. Acquaviva owned one himself, the Grazioli, now a world-class boutique hotel, its restaurant named after him.

For me, though, the most spectacular Frascati gardens are at the Villa Aldobrandini, where Anna Maria finally got her man. They were open to the public in 1745, just like today, and remain intensely romantic.

They luxuriate over an entire hillside, offering views to the horizon, but also unexpected pockets of privacy and many surprises: gargoyles and masks cut into the rock face, cascades and follies and topiary, and (as Casanova recalled) turf “banquettes”, perfect for what turned into a marathon alfresco defrocking session.

But someone had seen them. Thus began Casanova’s gradual expulsion from Rome and from the clergy. The city, he was told, “is small, and the longer you stay here, the smaller it gets”. Over dinner, drinks, at the theatre or at Mass, gossip was Rome’s main trade – and so it remains. Just as Casanova’s sins set tongues wagging, so the arrival of an English writer, daring to write about Casanova, opens unexpected doors for me.

One of them, at the Caccia Club, is perhaps the most exclusively aristocratic door in the city. I am taken there by Prince Oddone Colonna, whose ancestry stretches right back to Julius Caesar. We are to dine in Rome’s most elite club, its premises in the piano nobile of the Palazzo Borghese, lined with priceless antiques and signed photographs of the royals of Europe.

If I pass this test, I’ll be introduced to a Monsignor whose family owns one of the Venetian palazzos where Casanova lived. This is how things work in Italy, I am told.

We dine among liveried servants, 50ft under the frescoed ceiling of Pauline Borghese’s ballroom. I pass muster and get invited to a more churchly dinner the following Sunday. This is to be at the Bolognese, on Piazza del Popolo: a favourite of Tom Cruise and (on Sundays) cardinals.

It serves classically Italian food, including 18th-century favourites such as boiled meats with candied mustard fruits, and sorbets stuffed into walnut shells. A real taste of la dolce vita as lived by Rome’s more worldly clergy. I duly win an invitation to the Bragadin palazzo on my return to Venice.

Anna Maria knew Casanova would not be staying long in Rome, either. This may be why she chose to seduce the all-too-willing seminarian in the first place: she wanted a child and her husband was impotent. So while Casanova’s Roman holiday left him devoid of a career, it left Anna Maria with a daughter – not to mention some unusually picturesque memories of Frascati.

“To fit in in Rome,” Casanova later wrote, “one must be a chameleon... insinuating, a great dissimulator, often base, ostensibly sincere, always pretending,” which makes him sound quite scarred by his expulsion from the church. In truth, I think the cynicism he learnt in Rome stood him in good stead, as did his lessons in the art of lovemaking from an older woman in Aldobrandini’s heavenly gardens.

He remained forever attached to Catholicism, however, in particular that strand of Roman worldliness that knows the divine and corporeal are often bedfellows. “If a man is so unfortunate as not to have religion and loathes pretence,” he wrote, “he should leave Rome and seek his fortune in London.” Which is precisely what he did.

— Ian Kelly travelled as a guest of Italian Journeys and the Hotel Bernini Bristol

— Starting on Monday, June 24, Ian Kelly’s Casanova (Hodder £20) is Radio 4’s Book of the Week. To buy a copy for the reduced price of £18, including p&p in the UK, call The Sunday Times BooksFirst on 0870 165 8585

Travel brief

Getting there: airlines flying to Rome include British Airways (0844 493 0787, www.ba.com), Jet2 (0871 226 1737, www.jet2.com), Ryanair (www.ryanair.com) and Aer Lingus (0870 876 5000, www.aerlingus.com).

Where to stay: in Rome, the Hotel Bernini Bristol (00 39-06 488931, www.sinahotels.com) has a roof garden and lots of marble, with doubles from £252. More modest is the Hotel Campo de’ Fiori (06 6880 6865, www.hotelcampodefiori.com), where doubles start at £95. In Frascati, Villa Grazioli (06 945 4001, www.villagrazioli.com) has doubles from £166.

Casanova spots: Caffe Greco is at 86 Via Condotti (06 679 1700). Villa Aldobrandini is open to the public on request; tickets (£2.50) are available at the Frascati tourist office. Meals at the Bolognese restaurant (1-2 Piazza del Popolo; 06 361 1426; closed Mondays) cost about £40 a head. There are great outside tables, but these need to be booked.

Packages: Italian Journeys (020 7373 8058, www.italianjourneys.com) has three nights, B&B, at the four-star Hotel Piranesi, near the Spanish Steps, from £459pp, including flights from London and private transfers. Or try Kirker (020 7593 2288, www.kirkerholidays.com) or Citalia (0871 200 2004, www.citalia.com).

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