The attack added further urgency to a project already in the works: Macron has embarked on a controversial quest to change Islam in France --- with the goal of integration but also preventing radicalization.
He has said that in the coming months he will announce "a blueprint for the whole organization" of Islam. And those trying to anticipate what that will look like are turning their attention to Hakim El Karoui, a leading voice on how Islamic traditions fit within French culture.
It's hard to miss that the man who appears to have Macron's ear on this most sensitive of subjects cuts a similar figure. Like the president, El Karoui is an ex-Rothschild investment banker with an elite social pedigree who favors well-tailored suits, crisp white shirts and the lofty province of big ideas.
The latest of those ideas is this: that the best way to integrate Islam within French society is to promote a version of the religion "practiced in peace by believers who will not have the need to loudly proclaim their faith."
But if El Karoui is the model for how Macron envisions merging Islamic traditions and French values, the reform effort may end up stumbling along a rough road.
"He's disconnected from everyday Muslims, and he has legitimacy on the question only because he happens to be named Hakim El Karoui, and that's it," said Yasser Louati, a French civil liberties advocate and Muslim community organizer.
Since 2015, more than 230 people in France have been killed in terrorist attacks, most of them perpetrated by French and European nationals affiliated with or inspired by the Islamic State. Meanwhile, as many as 1,910 French nationals have gone to wage jihad in Iraq and Syria, according to the Soufan Center, which studies radicalization.
France has agonized over how to intervene --- what might work and what the government's proper role should be. Already, there have been missteps.
In 2016, the government opened the first of 13 planned "deradicalization centers" in a converted chateau in the Loire Valley, where at-risk youths volunteered for an immersive program in French culture and history. But the approach came under criticism for being too nationalistic, for taking the youths out of their homes and communities, and for relying on questionable science. Then locals began to protest having suspected terrorists next door. The Loire center closed in 2017, the concept declared "a total fiasco."
Macron has pursued an aggressive security policy. He permanently enshrined portions of an emergency security provision into French law last year. And he has continued a crackdown on mosques and imams associated with Salafism, an ultraconservative interpretation of Islam.
But he also seems invested in a broader, more cultural change to French Islam that transcends the question of national security. Since the 1980s, French presidents have attempted the same, but to no avail. The 40-year-old Macron is determined to succeed where others have failed. And he seems to be particularly intrigued by the ideas of El Karoui.
The son of a Tunisian Muslim father and a French Protestant mother, as well as a nephew of a former Tunisian prime minister, El Karoui identifies himself as a French Muslim. But he is not overtly religious.
El Karoui is in many ways the embodiment of the modern French elite ---- for some Muslims, perhaps too much so. In line with the strict interpretation of French secularism known as laicite, he opposes religious displays such as the veil - which he sees as a political tool that reinforces inequality between men and women and therefore runs counter to the core French value of egalite.
It wasn't until after the 2015 Charlie Hebdo attack, involving French Muslim suspects, that El Karoui began to research and write about Muslims in France --- a fragmented community of about 4.5 million, although the exact figure is unknown, given the French government's refusal to collect official statistics on race, ethnicity or religion.
The sum of his various stances are on display in a recent book, "L'Islam, Une Religion FranÃ§aise," an intervention that delighted much of the French elite because of El Karoui's call for Muslims themselves to take charge of their own community. "We need your mobilization," he writes in his conclusion.
"It is by committing yourselves that Islam will naturally become a French religion."
With a Macronian flair for procedure, he has approached the project as one might expect of someone with a background in finance. He has sought to quantify "Islamic consumption" in France and the "market share" of secular Muslims versus the Islamic State and Muslim Brotherhood. He writes about certifying halal produce "along the entire value chain." He is particularly interested in financial flows and wants to limit foreign support of mosques and imams - a proposal that Macron's recent statements suggest he may ultimately embrace.
"We made a strategic error in the past, when it was decided that Muslims were foreigners, and that Islam should therefore be funded by foreign states," El Karoui said in an interview at his Paris office.
He noted that Islam is the second-most-practiced religion in France and that the vast majority of Muslims in France, about 75 percent, are French-born. And yet money from Algeria, Morocco and Turkey supports the vast majority of French mosques and imams. "The idea of foreign financing has become completely insane," he said. He proposes funding French Muslim communal life through several domestic sources instead.
El Karoui's proposals have alienated some among France's more observant Muslims and those fighting Islamophobia. He has been criticized for accepting that radicalization is primarily a Muslim problem, which should be addressed within the ranks of the community. Yet many convicted terrorists have not been particularly religious.
Of particular issue was a 2016 study he conducted through the Institut Montaigne: "A French Islam is Possible," which, among other things, examined a link between Muslims and radicalization. The study was criticized on technical grounds: Some argued that its sample sizes were inconclusive, while others insisted that it asked leading questions.
"He has a pragmatic approach, and he wants to identify the issues and provide solutions," said Marwan Muhammad, a former director of the Collective Against Islamophobia in France (CCIF) and an anti-discrimination activist. "But what he defines as 'radical' has nothing to do with Islam."
El Karoui deflects the criticism, saying: "Among the biggest problems in the Muslim world, in the Arab world, is the incapacity to face the problem. It's always the fault of someone else. 'It's not our fault; it doesn't exist; it's a conspiracy; it's Israel's fault.' But what is the reality?"
He is less interested in other documented sources of Muslim alienation and radicalization, such as discrimination that may limit educational and employment opportunities, or France's prison system, where Muslims are disproportionately represented.
He is also reluctant to revisit first principles, such as the idea that the veil is a threat to the foundation of the French Republic. "That's a political reality," he said when pushed on the subject while speaking in the United States last year. "And we will not change France."
For a number of prominent French scholars, the idea of the state attempting to regulate an entire religion, to stop a small minority of extremists, is a losing battle. They also see it as a quasi-monarchical aspiration that has no place in a country committed to secularism.
"An apparent source of inspiration is the centralized integration of the Jews by Napoleon, but Napoleon was able to achieve that because he was the head of an authoritarian state," said Olivier Roy, an expert on French Islam and a professor at the European University Institute in Florence.
In 1808, Napoleon established by decree an organization known as the Consistoire Central Israelite, a body intended to serve as a central organizing authority for Jewish religious practice throughout France. A version of the organization survives today.
"It was the same with Louis XIV and 'Gallicanism,' " Roy said, referring to the view that French kings --- not the pope in Rome - should have ultimate authority over the control of the Catholic Church, as a means of control over the people. "That was normal at the time, but because the king was invested with divine right."
"A secular state," he added, "has no legal right to intervene in a religion. That would be unconstitutional. We hear so often talk about the formation of imams, about the idea of an Islam that accepts 'the values of the French Republic' --- but what are those, exactly?"
For others, what must also be addressed is what they see as the structural hypocrisy of the French state, which promises equality for all, but which often polices the way certain citizens can live their lives, as the recent controversies over the veil and the "burkini" swimsuit suggest.
French Muslims "have no place in public life," said Dounia Bouzar, an author and deradicalization specialist.
"We have to have communal liberty in the public space," she said.
• 10/25/17: The terrorist was shot dead but his case is still being litigated --- and for good reason
• 07/24/17: In France, the cold-blooded murder of an elderly Jewish woman ignites a debate over terrorism
• 07/17/17: Thank you, dear Donald': Why Macron invited Trump to France
• 05/22/17: Macron's 'radical centrism' sure looks a lot like conservatism