For “progressive” Catholics, the cappa magna is – appropriately, you may think – like a red rag to a bull.

I blinked in surprise when I saw the photographs. On June 18, the Archbishop of Liverpool – a diocese associated with the drab vestments and liturgy of the 1970s – processed into St Mary’s Shrine Church, Warrington, wearing a cape of violet silk whose train was so long that it stretched into the next postcode.

Archbishop Malcolm McMahon was sporting the cappa magna (“great cape”), a flamboyant pre-Vatican II vestment that hadn’t been seen in his neck of the woods for decades. But then this was a prelude to a pre-conciliar ceremony: he was ordaining two priests of the traditionalist Priestly Fraternity of St Peter (FSSP) using the Old Missal, something that hadn’t happened anywhere in England and Wales for over 40 years. He also celebrated a Tridentine Mass watched by the Bishop of Shrewsbury, Mark Davies, sitting in the choir.

The Vatican-sanctioned ordination of two priests who will never use the vernacular Missal was a remarkable event in England, unthinkable until recently. But it was the cappa magna that caught people’s attention. Most Catholics have never seen one. My only glimpse of this fabled garment was in 2008, when Cardinal Castrillon Hoyos celebrated the old liturgy for the Latin Mass Society at Westminster Cathedral. His cape was scarlet rather than violet, because he was a cardinal, but miserably truncated, scarcely longer than an ironing board. Apparently this was a “modern” cappa magna: the train was shortened before it was shunted away altogether.

Judging by this month’s photographs, the FSSP supplied Archbishop McMahon with the real thing. His cape was not just long but gorgeously plump and opulent, like a prop from a costume drama. The Borgias, perhaps. Which, for many post-Vatican II Catholics, is precisely the problem. They think the cappa magna – which can stretch to nearly 50 feet – reeks of worldly pomp. When Bishop Edward Slattery of Tulsa wore one in Washington DC a few years ago, they wrote to protest.

Bishop Slattery’s spokesman had a clever riposte. Did they not realise that the great cape is supposed to represent “the finery of the world, its pomp and prestige”? That is why the bishop (who may be a cardinal) is “publicly stripped of this finery and humbled before the congregation”. He has his silk choir vestments removed – and then re-enters in priestly garb.

Alas, the symbolism of vestments is one of many things that went out with the council. For “progressive” Catholics, the cappa magna is – appropriately, you may think – like a red rag to a bull.

And Catholics don’t come more self-consciously progressive than Professor Tina Beattie, director of a centre for research into religion, society and “human flourishing” at Roehampton University. She stuck a photograph of the robed Archbishop McMahon on her Facebook page and called for a caption competition. The suggestions were unfunny but as vicious as anything I’ve seen on the most rancid traddie blog – references to princesses, knickers and gay weddings, and all presumably written by people who support “equal marriage”.

As the Catholic writer Tim Stanley commented, “if the average liberal saw a Sufi or a Hindu doing this, they’d call it beautiful and sacred. When they see a Catholic doing it, they laugh. Why praise ritual in the faith of others but denigrate it in ours?”

Good question. One possible answer is that the FSSP ordinations represented a blow to the morale of liberals who assumed that Catholic worship would become ever more Protestant in style as “the spirit of Vatican II” tightened its grip.

Would they have been quite so upset if they had seen the ultra-traditionalist Cardinal Burke clad in the same robe? Perhaps not. He is easy to dismiss as representing a marginal faction in the Church that is hostile to this pontificate. Google Archbishop McMahon of Liverpool, however, and you will find plenty of photographs of the same man celebrating the English Mass in a plain chasuble between two ugly squat candles – the “look” favoured by his controversial predecessor, Derek Worlock.

The point is this: Archbishop McMahon truly understands Benedict XVI’s argument that traditional and stripped-down rubrics can co-exist in Catholicism, just as Latin and the vernacular can. And he also grasps that, contrary to expectations, the Old Missal has not lost its appeal. There are those who prefer it to the New; though small in number, they are growing as a proportion of practising Catholics.

We are coming up to the 10th anniversary of Summorum Pontificum, the great document in which Benedict XVI removed virtually all restrictions on the use of the traditional Missal. The startling and beautiful images from St Mary’s, Warrington, suggest that this particular genie is not going back into the bottle, no matter how many caption competitions Professor Beattie runs on her Facebook page.

Damian Thompson is associate editor of The Spectator and editorial director of the Catholic Herald