So, what is the good liberal to do? What is the good humanist to say? Boris Johnson’s anti-Muslim “jokes” were not a dog whistle, but a foghorn beckoning racists to his Steve Bannon-assisted leadership cause. But in the maelstrom he has deliberately caused, the risk is that liberals are silenced on criticising religion, Islam in particular. Are you for the niqab or for Boris’s racism? That’s a preposterous choice.

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As vice-president of Humanists UK, I have frequently criticised religion. Humanists never seek to ban anyone from practising any archaic superstition, but we do argue against the state sponsoring unreason or aiding the religious indoctrination of children. We are passionate free-speech advocates, standing with Voltaire’s (not quite) quote, “I disapprove of what you say, but I will defend to the death your right to say it”.

No one is gagging Johnson, though his defenders – all rightwing Brexiteers – leap to his defence, with Jacob Rees-Mogg calling this a “show trial”. Even Christine Hamilton clodhops on to the Boris bandwagon, by comparing the niqab to a KKK hood. Meanwhile, his Tory accusers are remainers – Damian Green, Dominic Grieve and assorted Cameronites. Everything in this mortally split party is forever inflected through Brexit.

Johnson is under “disciplinary investigation”, which could lead to suspension or even ejection. That’s an internal party matter to set the boundaries of Tory racism, just as Labour wrestles insanely over antisemitism. Will Johnson be sent for diversity training? How the party resolves this will entertain outsiders, but it’s not a national free speech issue. The Metropolitan police commissioner, Cressida Dick, rightly moved fast to scotch any question of the law intruding.

Herbert Morrison (Peter Mandelson’s grandfather) once said: “Don’t tell me what’s in the motion, just tell me who’s putting it.” In politics, the motives of the speaker matter most. A female comedian of Pakistani heritage writing jokes comparing the niqab to a dustbin liner and a letterbox inhabits another planet to an entitled Old Etonian princeling politician race-signalling to grub up votes by tormenting a tiny group of vulnerable women. Shazia Mirza shows how people of every heritage can mock extremes in their own culture and bring us all closer together. But the Johnson brouhaha divides: it has “emboldened mainly male perpetrators to have a go at visible Muslim women”, says the NGO Tell Mama, which reports rising incidents of abuse of women wearing niqabs, as well as hijabs, which don’t cover the face.

Johnson aims to normalise rudeness to Muslims – the easiest hate targets following Islamist terror attacks. In every society, racism is only dormant for as long as it stays socially unacceptable, but it’s a virus easily released by any contemptible politician willing to stoop so low.

The Muslim Council of Britain on Sunday wrote to the prime minister calling for no “whitewashing” in the Johnson inquiry, while reporting a new spate of hate mail. On the BBC Today programme, its spokesman Miqdaad Versi accused Johnson of “deliberately stirring up hatred”, as “a senior politician making a political move”. Indeed, even in the unlikely event that Johnson grovels out an apology, the damage is done – or for him, the racism prize is won.

But Versi said something else, too: he accused Johnson of “dehumanising Muslim women”. That was a step too far. What could be more dehumanising than the niqab and the burqa? Hiding a woman dehumanises her completely, turning a person into an anonymous thing.

On visits to Afghanistan I have been shocked to see how contemptuously women in burqas are treated in the street, often shoved aside by men as obstacles in the way. The burqa doesn’t give women more respect, but less. Visiting secretive women’s groups in Kabul, I heard them voice hatred for the compulsory garment they discarded in the hall. Nor, as I wrote at the time, was the burqa a Taliban fetish; it was also imposed by the Northern Alliance “liberators” for whom we invaded.

Outside Boris Johnson’s constituency office last week, a small group of niqab-wearers demonstrated with signs reading: “My dress, my choice”. Fair enough. Even cake-and-eat-it Johnson professed their right to wear what they liked, while playing the race card. But the few women who are educated, liberated and free to choose the niqab as a religious symbol of an extreme fundamentalist creed are almost certainly a minority. Many whom Johnson has made more vulnerable to attack will be undefended, non-English speakers, obedient to men and a culture over which they have little choice, like the Muslim women I have met in English classes.

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Religions have always branded their identities by restrictions on women. Christianity, Judaism, Islam and others all set out with extreme rules proclaiming a disgust of unclean women’s bodies, with ritualised baths, head-shaving, denying abortion and contraception, arranged marriages, purdah, churching of new mothers, and barring women from priesthoods. Inside extreme cults and sects, abuse of women is almost inevitable.

Only when the hot phase of a faith cools down to homogenised Thought for the Day blandness do women begin to breathe freer. But liberation takes bravery, as with the 50 Muslim women in Scotland launching an equality campaign, Scottish Mosques For All, complaining of no women on mosque committees, nor women speakers, no creches, not even prayer facilities. But as yet they dare not speak to the media, only launching a Facebook questionnaire.

Humanists campaign against religions dividing communities. I doubt Boris Johnson will join our campaign against faith schools, as his party seeks to extend them. For community cohesion, niqabs matter a lot less than this most irreligious of countries allowing a third of state schools to be run by religious groups. But his intention is to sow discord, on which he hopes to ride to triumph.

• Polly Toynbee is a Guardian columnist