— When Prime Minister David Cameron unveiled his plan to legalize same-sex marriage last year, Britain’s highest Roman Catholic cleric took to the national pulpit. Cardinal Keith O’Brien decried a “tyranny of tolerance,” calling gay marriage “grotesque” and saying no secular government had the moral authority to legalize such unions.

On Monday, O’Brien, one of the church’s most strident voices against homosexuality, abruptly stepped down amid allegations of “intimate” acts with priests. His fall underscored perhaps the greatest challenge for the Roman Catholic hierarchy as it moves to elect a new pope: regaining its own moral authority.

Nowhere is that more true than here in Europe, the continent where the global church is losing the most ground. The taint of scandal here was far fresher than in United States, with a new wave of revelations of sex abuse by European clergy emerging in 2010. Since then, evidence suggests that a long and gradual exodus from Roman Catholic pews has only accelerated, with tens of thousands of Europeans abandoning the faith.

Through a spokesman, O’Brien has denied the allegations. But to some, his surprisingly swift departure heralded a new age of zero tolerance at a Vatican that once methodically stood by the princes of the church through thick and thin. Following a whirlwind of events, Pope Benedict XVI accepted O’Brien’s immediate resignation on Monday — only one day after allegations surfaced by four men, including three current priests, who accused the cardinal of initiating “intimate” and inappropriate advances.

But the nature of the allegations against O’Brien has already led others to call them — if proved true — an example of the kind of hypocrisy that is eroding the church’s influence, particularly on the globe’s most socially liberal continent. Fresh scandal now, observers say, could undermine the church’s current battle to restore its voice in the region, which it has waged by rallying against a bevy of liberal causes from legalized abortion in Ireland to gay marriage in France.

“It’s not possible to limit the damage anymore, they’ve just got to repent and open the door to all of the voices they have tried to silence and exclude,” said Tina Beattie, professor of Catholic studies at the University of Roehampton in London.

Though it remains unclear whether the allegations against O’Brien constitute a crime in Britain or fall within statutes of limitations, church officials have said he is seeking legal counsel. When the claims became public Sunday, O’Brien skipped a key Mass in his diocese in Scotland, where he has been head of the church since 2003.

On Monday, the cardinal, 74, shied away from any direct reference to the allegations, saying he was stepping down and recusing himself from the conclave to pick the next pope to avoid casting a shadow on the proceedings.

“For any good I have been able to do, I thank God,” O’Brien said in a statement. “For any failures, I apologize to all whom I have offended.”

His resignation comes as the church — while growing virtually everywhere else in the world – is facing an uphill battle in Europe, where the church’s own census showed that the number of Catholics was falling even before the 2010 revelations about sexual abuse.

“The Catholic Church will not ordain women, will not marry men, but it needs to work out how to talk about its beliefs to people who have heard it before and rejected it, so the people in Europe can listen again,” said Jack Valero, spokesman for Catholic Voices, an association of British Catholics. “At the moment, the P.R. is bad.”

Take, for example, Ireland — long one of the most devout Roman Catholic countries in Europe.

After a wave of church scandals, the government there is now in the midst of rolling out a plan that would initiate a gradual shift away from having church authorities act as the official patrons of virtually all public schools — something that would limit teachings of Catholic sacraments and theology.

“The country in which religion has strived most strongly is where church and state are separated,” said Ruairi Quinn, Ireland’s education minister, referring to the United States in explaining Europe’s drift away from the church. “The countries where religion is languishing either through neglect or indifference are those European countries which previously were dominated or controlled by the Catholic Church such as Portugal, Italy, Spain, not to mention Ireland.”

After keeping a relatively low profile following abuse scandals, the church has recently sought to raise its voice in Ireland and other parts of the continent, citing what church officials call the need to confront the rise of “aggressive atheism” and more-liberal Protestant faiths.

It has, for instance, strongly inserted itself into the debate over abortion following the death of a woman in Ireland last November. Doctors allegedly refused to terminate her pregnancy for health reasons “in a Catholic country.”

Church officials there have spoken out against a government pledge to pass laws this year that, for the first time, would unambiguously permit abortions under limited circumstances. A leading Irish bishop took to the airwaves last month to denounce the rise of a “culture of death,” while a high-ranking Vatican official suggested in a Catholic newspaper that Irish priests could withhold Communion from politicians and other citizens who support abortion.

“The church has been almost out of the social debate up until quite recently because of the [sex abuse] scandals,” said the Rev.
Brian McKevitt, editor of Alive, an Irish Catholic newspaper that has been running antiabortion editorials. “But if the church does not stand up for this, against abortion, then it has no function left.”

In Britain, which officially broke with the Vatican in the 16th century, the response to O’Brien’s resignation Monday bordered on shock. Born in Northern Ireland on St. Patrick’s Day, O’Brien rose to be a pillar of Britain’s still substantial Catholic community, becoming head of the church in Scotland and viewed by religious conservatives of multiple faiths as a devout traditionalist.

He once described abortions as “massacres” and said an embryo stem-cell research bill would allow for “Nazi-style experiments.” Though he became a leading voice against gay rights, his positions were not always easy to define. Last week, the cardinal gave an interview in which he suggested the church might need to revisit the question of clerical celibacy.

O’Brien was already scheduled to retire next month. By officially accepting his retirement Monday, the pope effectively accelerated his exit. Many in Britain, including Catholics, however, were still looking to O’Brien to offer a more detailed response to the allegations, published Sunday in the British newspaper the Observer.

The report cited four men who approached the Vatican’s emissary in London a week before the pope’s Feb. 11 resignation with the allegations against O’Brien.

“The church is beautiful, but it has a dark side and that has to do with accountability,” the paper quoted one of the men — who have remained anonymous — as saying. “If the system is to be improved, maybe it needs to be dismantled a bit.”

Eliza Mackintosh contributed to this report.