Four hundred and fifty years ago, on February 25, 1570, Pope St Pius V issued the bull Regnans in Excelsis excommunicating Elizabeth I, Queen of England.

He did not mince his words. Since “He who reigns on high” (Regnans in Excelsis) had appointed Peter and hence every successor “ruler of all peoples and kingdoms”, Pius now declared “the pretended queen of England and servant of crime” excommunicated and deprived of her “pretended title [of queen] and all lordship, dignity and privilege”.

The charges against her were formidable. She had reduced England to “miserable ruin” by promoting heresy, filling the royal council with heretics in place of the “nobility of England”, abolishing the Mass, clerical celibacy and Catholic prayer and practice. Having “monstrously usurped the role of supreme head of the Church in all England”, she had “ejected bishops and priests from their posts and thrown them into prison, where many have long languished and died”. She had refused to allow papal nuncios to enter her realm and imposed an unjust oath on her subjects. And so on.

But the biggest sting was in the tail. Since she was now an excommunicated heretic, her subjects were absolved from “any oath, duty or fealty made or due to her”. Moreover, all her subjects were charged “not to dare obey her orders, mandates and law – under pain of excommunication themselves”.

Because much of English historiography has been imbued with Protestant Whiggery, Pius’s treatment of Elizabeth has often been portrayed as a throwback to the high papalism of “megalomaniacs” such as Innocent III or Boniface VIII; an unjust deed because Elizabeth had never really been a Catholic; a vain gesture which inevitably provoked repression.

And – so the argument goes – if that bull had not been an embarrassing display of papal arrogance, why did Pius’s successor (Gregory XIII, the greatest pope of the 16th century) promptly try to bury it by ruling that it would not bind anyone until it could be “activated”? That was a typical Roman face-saver. Gregory surely knew in his heart that his predecessor had grossly overstepped the mark.

But papal excommunication was still alive and well in the 16th century – as Luther and later Henry IV of France could attest. England’s Henry VIII had only narrowly escaped it at the hands of Paul III. Indeed, Gregory XIII readily brought the bull out of storage in 1588 when the Spanish Armada threatened to unseat the Queen.

Nonetheless, Regnans in Excelsis was a mistake – although not because it was undeserved.

Elizabeth had been “bastardised” and excluded from the succession by an Act of Parliament, but Catholic Mary had named her as her successor on her deathbed. In truth, there was no credible alternative – and besides, during her half-sister’s reign, Elizabeth had apparently undergone a conversion and become a regular Mass-goer, even asking Mary for guidance on what good Catholic books she should read.

So Mary probably died believing that her successor would keep England Catholic. The goody two-shoes act had worked.

Elizabeth had been crowned by a Catholic bishop, the amiable Owen Oglethorpe of Carlisle, the only surviving archbishop (Heath of York) having refused. (Oglethorpe quickly regretted what he done – and, like Heath, was soon deprived and in confinement).

Pius IV, St Pius V’s predecessor, had apparently been ready to do a deal with Elizabeth. She was more than ready to play him along and he nearly fell for it.

But Elizabeth was not really open to a deal. She had sacked all the surviving bishops. Several were in the Tower of London and the notorious Marshalsea prison in Southwark. The Mass had been abolished. Her celebrated “via media” was a middle way between one form of Protestantism and a less radical one, not a half-way house between Geneva and Rome.

The majority of her subjects conformed – or drifted along – with varying degrees of enthusiasm and conviction. But some, though few in number, did not. They included important members of the nobility, notably the young and admirable Earl of Northumberland, Thomas Percy (of whom more soon).

Certainly a growing number of English priests and lay people who had fled their homeland did not submit. They included William Allen, a leading figure in a group of vigorous English Catholic exiles in Louvain, Belgium, and Thomas Goldwell, the last Catholic bishop of St Asaph, who had made a quick escape soon after Mary died and found his way back to Rome.

Exactly how the story developed thereafter is not clear. Elizabeth did not want to make martyrs (for political rather than moral reasons) and was content to allow her coup d’église to settle at its own pace. The drip, drip of bloodless persecution (fines for non-attendance at church, exclusion from universities and public offices, etc) would surely cure all.

For the likes of Thomas Percy and Thomas Goldwell, however, it was increasingly clear that there had to be a bloody showdown. With the accession of Pius V in 1565 that became a possibility.

But Pius had more than a little to contend with already: enforcing the wide-ranging decrees of the Council of Trent, combatting Protestantism (especially in northern Italy) and organising the Holy League against the ever-advancing Ottoman Empire. The Holy League was eventually to win the famous Battle of Lepanto in 1571 – and thus save Rome itself from going the same way as Constantinople in 1453. The affairs of remote England would have to fight for a place on his agenda,

It was not until 1568 that he could give them serious attention. Persuaded at last (almost certainly by Allen and the firebrand Goldwell), he prepared to excommunicate Elizabeth. But he would not do so unless she was likely to submit, or firm plans were in place to depose her. Otherwise the excommunication would be an empty gesture and entail serious loss of face. (Henry VIII’s excommunication was never finally launched, almost certainly because there was no longer much chance of a successful rebellion against him).

Accordingly one Nicholas Morton, sometime canon of York who had fled his homeland and was now a protégé of Goldwell in Rome, was sent secretly to England to consult the likes of Thomas Percy to discover how the proposed excommunication would be received; that is, whether it would be accompanied by an uprising.

Morton reported that it would be widely welcomed. Indeed, he had been begged to get immediate papal support for a revolt that was already being planned.

Exactly what happened next is, once again, unclear.

Pius did not act promptly (perhaps because he was engrossed in so many other urgent issues). So the two earls (Northumberland and Westmorland) who were to lead the 1569 rising wrote to him on its very eve, begging for his blessing. Pius replied with astonishing speed, promising to provide financial and spiritual aid and to declare Elizabeth a heretic.

The rebellion was soon under way. It swept across northern England. Mass was said once again in Durham Cathedral. Thousands were joining in. Had not Elizabeth’s chief captain held York – and, more importantly, had the promised papal excommunication arrived – the rebellion could have surged southwards and probably toppled Elizabeth.

But inexplicably Pius had not yet delivered his bull. Why? We do not know. Not until February 10, 1570 – to observe due process (and no doubt believing that the rebellion was still alive) – did he take final evidence from 10 English residents in Rome to confirm Elizabeth’s guilt. At last, on February 25, the bull was released. And to enable it to be circulated faster, the bull itself authorised production of notarially attested copies anywhere.

The bull probably did not arrive in England until mid-March. Famously, a heroic young Londoner, John Felton, acquired a copy and nailed it to the door of the Bishop of London’s house by St Paul’s. He was executed in St Paul’s courtyard for his deed a few weeks later – all this greatly (and unwittingly) helping to spread news of the bull.

But it was too late. The rising had long since collapsed and a bloody revenge launched. Many hundreds were slaughtered. The admirable Thomas Percy, now beatified (how many other English earls have been raised to the altars?), was eventually executed in York, after long imprisonment, two years later.

There is a final element to be added to this already complicated story. In May 1568 there had arrived in Workington in a fishing boat a bedraggled Mary Stuart, Queen of Scotland. Having escaped prison and then been defeated in battle at home, she now sought refuge in the land of her Tudor kinswoman.

For Elizabeth, she was an acute embarrassment. What was she to do with this unwanted royal visitor? For Catholics planning rebellion, she could become exactly the trophy they needed. And she could call up Scottish Catholic support. Indeed, it was suggested, if the pope would pardon Scottish lairds who had fallen into heresy and acquired Church properties, many more would come to her aid. And she might end up on the throne of England.

But this most fatal of femmes fatales was also an embarrassment. Twice briefly married to a king (of France and then Scotland), she was implicated in an explosion of gunpowder which carried off her third husband, and she had subsequently “mislaid” a fourth.

She was seemingly a Catholic – but if a marriage to the Protestant Duke of Norfolk, which would greatly strengthen her chances of replacing Elizabeth, could be arranged, her faith was negotiable.

If Paris was worth a Mass to Henry IV of France, London was perhaps worth a Book of Common Prayer to her.

Did Pius know something about her? Was that why he hesitated to destroy Elizabeth? Could that austere pope have ever backed an adulteress who had married the man who (with her connivance) had murdered her previous husband?

Regnans in Excelsis was a grievous error – not because it was undeserved, but because it came a few weeks too late to sanction the rebellion which it would have legitimised, and would have thus brought England back into the Catholic fold. Instead, it unleashed decades of often bloody persecution.

In these ecumenical days we often rather forget those who suffered “come rack, come rope” and “in spite of dungeon, fire and sword”. But their heroism hugely inspired the remarkable renewal of English Catholicism during the 19th century and well into the 20th. Apparent defeat eventually yielded rich new life.

God indeed draws straight with crooked lines.

Professor Jack Scarisbrick is a historian and emeritus national chairman of Life