THE first rule of Pope Fight Club is that thou should take a vow of silence about Pope Fight Club.

Not a very good joke perhaps, but one of many doing the rounds after Pope Francis's extraordinary intervention into the Charlie Hebdo affair. I fear we may not be making jokes like that for much longer if the Islamic Inquisition get their way.

The Pope said, in effect, that if you curse religion you can expect a punch on the nose. "You cannot provoke," said Pope Francis. "You cannot insult the faith of others. You cannot make fun of the faith of others." Some say the Pope was "only joking" and that he also condemned terrorism. But the message seemed pretty clear: respect - or pay the consequences. Anyway, it was a profoundly irresponsible remark and offensive to the victims of the Charlie Hebdo massacre.

I don't like to quote myself, but I predicted in wake of the events in Paris that Roman Catholics would soon be calling for immunity from mockery, just as pro-Islamists in the West have been saying that ­cartoonists have no right to use ­freedom of speech to mock "minorities". We can all play that game.

If we allow Islam to be privileged, then other religions will demand the same right of exclusion from critical debate. We will be back to the days when people tried to outlaw Monty Python's Life Of Brian because it was offensive and blasphemous. Imagine Monty Python's Life Of Muhammad: "He's not a prophet, he's a very naughty boy." Film-makers have been murdered for less.

Well, I'm sorry, your Popeness, but we can, we should, and I hope will continue to mock, criticise and ­ridicule religion and all those ­irrational faiths which profess peace but somehow end up ­fighting with each other over obscure matters of doctrine; which impose arbitrary moral codes and condemn unbelievers and sexual minorities as sub-human. The difference is that, unlike the zealots, we humanists accept the right of even the faithful to express views we find abhorrent.

The aftermath of the Charlie Hebdo attack has been deeply troubling for those of us who still believe in freedom of speech: a right hard won over 200 years and one which appears to be under threat from a combination of terrorist violence and a misguided political correctness which equates ridicule of Islam with ridicule of an ethnic group.

I keep hearing how we should avoid "Islamophobia", as if this were some kind of racism. It is not. I am profoundly anti-Islam but that does not make me a racist. I am also profoundly anti-­Christian, anti-Hindu, anti-Catholic, anti-Protestant, anti-Moonie and anti-Scientology. These are not peoples but belief systems. And they all claim a monopoly of religious and moral truth, which is why they can rarely coexist with rival faiths.

There are many aspects of ­religion that I find profoundly offensive and objectionable, but the only thing we should ban is intolerance. And I don't just mean Islamic intolerance, whose exponents in Saudi Arabia are so defensive of their faith that they sentenced a blogger to 1000 lashes for mocking their religious police. Pope Francis has reaffirmed the rejection of contraception and abortion on the usual grounds of Biblical dogma. These things should be challenged, and if necessary, wilfully mocked.

But it is said by misguided ­multiculturalists that we shouldn't indulge in "racialised baiting", as The Guardian's Seamas Milne described the Charlie Hebdo cartoons. We should not "disrespect the oppressed by disrespecting their Prophet" as someone on Twitter put it. People whose views I normally respect kept telling me last week that as "a privileged white middle-class male I do not have the right to say what is and what is not racist". Well, I'm sorry, but I do - and ­mockery of religion isn't.

Anyway, there are laws about these things. On the BBC's This Week, French-Algerian journalist Nabila Ramdani said the Charlie Hebdo cartoons were "incitement to racial hatred" and had "caused the violence". But France already has some of the most restrictive laws on racial offence in Europe (far too restrictive, in my view), against which the magazine's cartoons were tested in court - and were found to be acceptable. Incitement to racial hatred isn't what we are talking about here.

People like Nabila Ramdani don't accept non-Muslims' right to make any representation of the Prophet - even the ­remarkably ­conciliatory and graceful one on the cover of last week's Charlie Hebdo, where he says "all is forgiven". What irony that the anti-clerical satirists of Hebdo succeeded in being more true to Islamic morality than many of its media apologists. For, like Jesus, the Prophet Muhammad is supposed to have advised followers to turn the other cheek, not kill detractors.

Yet Ramdani - a supposedly liberal Muslim who doesn't wear a hijab - insisted: "You can't be a bit pregnant and you can't be a bit blasphemous." In her world, depiction of the Prophet is forbidden and should be punished not by death but by legal sanction. An astonishing number of people in the UK seem to agree with her.

I am afraid the terrorists have won the first round of this ­struggle. Newspapers here are refusing to republish the Charlie Hebdo cartoons on the grounds of "offence". Soon depictions of the Prophet will be removed from books on religion. Critical works like Salman Rushdie's The Satanic Verses will disappear. Comedians like Shazia Mirza, a Muslim who has mocked sexism in Islam, will be silenced by the BBC "so as not to give offence". A deep chill has descended on our intellectual life.

Restrictions of freedom of speech, of course, pre-date the Paris attacks. Islamic fundamentalists follow social media and have noticed the West's "culture wars" and the way they have led to self-imposed interdicts on expression on grounds of race, gender and identity. Muslims complain that we indulge in double-standards by outlawing inflammatory remarks about gays, Jews and Afro-Caribbeans. ­Measures like the Offensive Behaviour at Football and Threatening Communications (Scotland) Act 2012, which attempt to ban ­sectarian songs, are cited by Muslims as proof that we outlaw one kind of religious hatred but not others. And they are right. The sooner this illiberal act is repealed the better.

There has been a debate this week about Flower Of Scotland becoming the Scottish national anthem. Applying the logic of the Offensive Behaviour Act, it is inconceivable that we should allow this celebration of military defeat of the English to be allowed to be sung in sports grounds where Irish Rebel songs are proscribed.

But ultimately, you can't police thought. I disagree with the law against Holocaust denial in Germany for precisely this reason. The people who try to deny the existence of concentration camps and the Nazi final solution condemn themselves by their utterances and should not be accorded the dignity of being outlawed.

We are entering a new age of unreason. Freedom of speech is the paramount expression of ­freedom itself. If people are not free to think and debate, to challenge and ­ridicule, then they cease to be truly human. We become automatons.

For years, I have described myself as a "lapsed humanist". It seemed to me that religion had retreated from the public sphere into the confessional and we had seen the back of blasphemy laws and clerical invasions of intellectual life. That the Enlightenment ­principles of Thomas Paine's The Age Of Reason were now so ingrained in our culture they were inviolable.

After last week, I'm beginning to think that the battle against ­obscurantism has to be fought all over again. No-one expected the Islamic Inquisition.