It is perhaps a small comfort in the dark day that follows his murder, that the editor of Charlie Hebdo, Stephane Charbonnier, fulfilled the defiant promise he once expressed in an interview: “It probably sounds a bit pompous but I’d rather die standing than live on my knees.” Thus he died, along with 11 others, as the editor of a magazine that had the courage to subject Islam to the same derision as its other targets, including the French political elite, Catholicism and feminism. Mr Charbonnier and his colleagues, among them some of France’s best-loved cartoonists, have paid a heavy price for their refusal to be intimidated from satirising Islam. Their murder is France’s 9/11, an attack on an equally totemic target. In a country in which free expression is valued not least as an expression of the secularism at the heart of its republican principles, this is an attack on what makes France French.

Naturally there has been a near-universal outpouring of grief and solidarity, not just in France but in Trafalgar Square last night and in cities throughout the world. We are all Charlie Hebdo now. The expressions of revulsion have been shared by Muslim leaders and the Arab League. This episode is plainly not quite in the same category as the controversy over the Danish cartoons in 2006, when the attacks on the cartoons and the magazine that published them attracted support throughout the Muslim world, including demonstrators in London. Yet the willingness of Charlie Hebdo to republish those cartoons is one reason why its editor is dead.

For all these declarations of support, it is perhaps too early to declare that Western proponents of free speech will never be subdued by the threat of violence. It is worth asking whether, if Salman Rushdie were to write another version of his Satanic Verses about a controversial section of the Koran now, he would find a publisher and outlets willing to sell and promote it. Would any British institution broadcast even a dispassionate and historically valid critique of the prophet of Islam? It remains a potentially risky move.

It would be foolish and wrong for any of us to seek to demonstrate our independence of mind by gratuitously giving offence to Muslims. Yet freedom of speech, crucially, includes the freedom to offend — so long as such expression does not condone violence. Whether Muslims find depictions of Mohammed or such criticism offensive or not is immaterial. Freedom of speech is indivisible: it protects Muslims’ right to free expression as surely as it does those who criticise or lampoon Islam.

Yesterday’s murder was new, in the West, in its planned, military-style character, which suggests that the assassins may have benefited from military training. Their claim to represent al Qaeda is at least plausible. But it is also part of a disturbing trend towards growing militancy among radicalised French Muslims. The growing intimidation by Islamic fundamentalists of Jews in France has resulted in an exodus of thousands of them to Israel. There have been troubling attacks on Jewish schools and on a Jewish museum in Brussels. Plainly the killers are utterly unrepresentative of the majority of peaceable French Muslims — but that community is also manifesting growing radicalisation at the edges. It is significant that France has seen an estimated 700 of its citizens recruited to fight for the jihadis of the so-called Islamic State in Syria and Iraq.

London, with its multi-ethnic character, feels a particular solidarity with Paris at this time. London is home to many thousands of Parisians, and the French capital is a place beloved of Londoners. We have the utmost sympathy with the city and huge admiration for the independence of spirit of Charlie Hebdo. We can only raise our pens to salute its courage and hope it continues. Nous sommes tous Parisiens maintenant.