The turning point in the 2015 federal election campaign in Quebec came in mid-September, a month before voting day, when the Federal Court of Appeal struck down a Conservative government ban on face coverings at citizenship ceremonies. For New Democratic Leader Tom Mulcair, it was the moment of truth that ended his party's long run atop the polls in the province it had swept in 2011.

The NDP had come face-to-face with its own two solitudes.

The Quebec left is uncompromisingly secularist. While it supports freedom of religion, it believes that visible manifestations of faith are to be discouraged in the public sphere, lest they impinge on the separation between church and state. Quebeckers fought hard to throw off an oppressive Catholic Church and see any religious accommodation by the state as a threat to the gains of the Quiet Revolution. More recently inspired by France's secularist approach, the Quebec left supports strict limits on where and when religion can be practised.

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The niqab decision sowed deep unease within the NDP's Quebec ranks. And Stephen Harper's Conservatives and the Bloc Québécois under Gilles Duceppe seized on the ruling – which cleared the way for a 29-year-old niqab-wearing Pakistani immigrant to take the oath of citizenship in time to vote in the Oct. 19 federal election – to accuse Mr. Mulcair of holding different positions on the niqab depending on which part of the country he was campaigning in. The NDP Leader finally came out in favour of the court decision, but not before several days of waffling. On Oct. 19, his party went from 59 to 16 seats in Quebec.

The niqab saga now haunts the NDP as the party prepares to elect Mr. Mulcair's successor. If Quebeckers are uncomfortable with a private citizen displaying her religion, what are the chances they would ever elect a prime minister whose faith – a minority one, to boot – defines him?

Spare us the old trope about Quebeckers being more xenophobic than other Canadians. NDP leadership candidate Jagmeet Singh's Sikh religion is likely no more a deal-breaker among voters in rural Quebec than among those living in non-urban areas elsewhere in the country, which is to say xenophobia exists everywhere. But what really separates Quebeckers from other Canadians is their antipathy toward public manifestations of religion – unless of the anodyne "Catholic cultural heritage" variety. In the rest of Canada, tolerance of religious diversity goes hand in hand with support for multiculturalism. In Quebec, secularism and interculturalism rule.

Secularism requires religious neutrality on the part of the state. Interculturalism posits the integration of all citizens into a "common culture." In 2013, the then Parti Québécois government invoked both principles in tabling its Charter of Quebec Values, which would have prohibited public-sector employees from wearing "ostentatious" religious symbols. Of course, the Charter was mostly a politically driven ploy to win over rural voters. But it was grounded in principles championed by the Quebec left.

In the end, Quebec progressives saw the Charter for what it was – a cynical PQ attempt to stigmatize Muslims for political gain. But that hardly ended the debate over religious accommodation in Quebec. Even the Liberal provincial government now advocates forcing women to show their faces when dispensing or receiving public services. Even the far-left Québec Solidaire supports banning persons in positions of authority from wearing religious symbols.

Not surprisingly, then, the concept of a turban-wearing prime minister is a hard one for many Quebeckers to swallow. The idea of the country's highest elected representative, and the face of its government, openly displaying his religion is an affront to their secular values. For the most part, Mr. Singh has shown his Sikh faith to be perfectly compatible with the values of equality and social justice all New Democrats champion. But the Ontario MPP has also let his faith influence his policy positions, notably in his opposition to his province's sex-ed curriculum.

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"Ontario is a diverse province and we must respect the diversity of beliefs when it comes to educating our children," the MPP for Bramalea-Gore-Malton said in 2015. That argument is a Trojan horse for Quebeckers, who frown on religious exceptions in public schools. The prospect of Mr. Singh being beholden to large Sikh populations in Ontario and British Columbia won't help him in Quebec, either.

There should be no conflict between a prime minister's faith and his or her ability to uphold secular values. But without winning Quebec, Mr. Singh may never get a chance to prove it.