In more superstitious times, priests and magistrates led witch-hunts against those accused of heresy and consorting with Satan. In our supposedly more enlightened age the authorities are staging a witch-hunt against a north Belfast pastor who damned Islam as "Satanic" - the modern equivalent of heresy.
Pastor James McConnell is facing prosecution - and up to six months in prison - simply for expressing a belief that some others might find offensive. In this, the maverick preacher has become an unlikely symbol of the new free speech wars.
During a sermon in his Whitewell Tabernacle last year, Pastor McConnell suggested that "Islam is heathen, Islam is Satanic, Islam is a doctrine spawned in Hell". Not the sort of woolly language we in Britain are used to hearing from the anodyne Anglican Church, perhaps, but no more than the honestly expressed view of an evangelical Christian. Such on-the-offensive honesty, however, can be a criminal offence these days.
After the sermon was streamed on the internet the police knocked on Pastor McConnell's door. More than a year later the authorities have finally charged him under the 2003 Communications Act with being responsible for an electronically-communicated message "that was grossly offensive". Pastor McConnell is due in court to answer charges in August and intends to plead not guilty.
Note the language of this law; an insidious menace to freedom of expression. The pastor has not been accused of threatening violence via the internet, or of sending indecent, or illegal, material. His alleged crime was to express a "grossly offensive" point of view. These days that can be deemed the grossest offence of all.
The primary threat to free speech in the UK comes not from jackbooted State censorship crushing political opposition. It comes, instead, from a smothering climate of conformism, where the message of the secular sermonisers is "You Can't Say That!"
Everybody in public life supports free speech "in principle". "Principle", however, is another country. Back here, in practice, they all have a "but…" with which to restrict opinions that offend their sensibilities.
Official censorship of offensive speech under laws such as that Communications Act mean that more people are now arrested and imprisoned in the UK for what they think, say, or write than at any time since the 18th century.
Then there is the unofficial censorship practised by online "Twitter mobs" of ban-and-boycott-happy zealots, who want not just to criticise but to censor those who infringe their strict speech codes.
These witch-hunters, wielding buzzing smartphones rather than flaming torches, recently hounded Nobel Prize-winning biochemist Tim Hunt out of his profession for making a daft joke about women in science.
The dual menace of official and unofficial censorship is creating a stultifying atmosphere of self-censorship, where the pressure is on to withdraw any offending remarks and apologise at the first sign of a wagging finger.
It is no longer the Churches and traditional conservatives leading demands for censorship. Today the You-Can't-Say-That crusaders are supposed liberal and radical-minded campaigners, who claim to want to protect "vulnerable people" from offensive and hurtful opinions. Thus, they demand the likes of Pastor McConnell are punished for "hate speech".
Free speech, however, is an indivisible liberty to be upheld for all or none at all. To defend it only for those you agree with is "me speech" rather than free speech. Once we invite the authorities to start judging which opinions are fit for public consumption, whose views will be safe?
It is in the spirit of supporting open debate free from State interference that a Muslim academic, Dr Al-Hussaini of the Westminster Institute, has sworn to go to jail with Pastor McConnell, insisting that "in a free and democratic society, we enter into severe peril when we start to confuse what we perhaps ought or ought not to say, with what in law we are allowed to, or not allowed to, say".
It is in the same spirit of defending unfettered free speech that an atheistic, libertarian Marxist such as me defends Pastor McConnell's right to call Islam Satanic - and Dr Al-Hussaini's right to reply in kind.
Nobody has to agree with the pastor's views in order to defend his freedom to express them, any more than we had to find Charlie Hebdo's cartoons hilarious to stand for its right to be offensive.
Charlie's native France is the land of Voltaire, the revolutionary writer whose history-making views on free speech and tolerance are famously summarised as "I disapprove of what you say, but I will fight to the death for your right to say it".
Today, by contrast, we in the West live in the age of what I call the "reverse-Voltaires", who turn that principle on its head and declare, instead, that "I know I will detest what you say and I'll fight to the end of free speech for my right to stop you saying it".
Whatever anybody might think of Pastor McConnell's beliefs, his determination to stand by them stands out in the age of the reverse-Voltaires. He told the Belfast Telegraph that, while he has apologised for any hurt feelings, he makes no apologies for denouncing Islam as Satanic and would rather go to jail than withdraw.
He also said that he would "defend the right of any Muslim cleric to preach against me, or Christianity". Thus did the firebrand evangelical display a better grasp of true tolerance than the allegedly enlightened authorities.
This is the sort of opinion that gets the likes of Pastor McConnell branded as "Islamophobic" - a cheap device which equates an honest difference of beliefs with a form of psychiatric disorder.
And, of course, there is no point debating with irrational "phobics" - far better to get them and their dangerous opinions into a straitjacket and padded cell, figuratively if not literally. The pastor's lawyer, Joe Rice, has vowed to turn the case into a landmark trial "in defence of freedom of speech and freedom of religion". He might find some unexpected allies among the giants of the historic struggle for free speech.
He could call Baruch Spinoza, the Dutch Enlightenment thinker condemned as a heretic who, almost 350 years ago, set the standard that we are still struggling to meet: "In a free state, every man may think what he likes and say what he thinks."
Radical British author George Orwell would make another fine witness. At the end of the Second World War, Orwell wrote, in a preface to his novel Animal Farm, that: "If liberty means anything at all, it means the right to tell people what they do not want to hear." His publisher refused to include such heresy.
The pastor could also find support from the famous Justice Oliver Wendell Holmes of the US Supreme Court, who, in 1929, declared the most important principle of a free society to be "the principle of free thought - not free thought for those that agree with us, but freedom for the thought that we hate".
To which the witch-hunting zealots of today would surely respond: "Freedom for the thought that we hate? You can't think that!"