Jesus would be banned from preaching at universities today, according to an eminent Oxford professor.

Speaking at the Hay Festival, Timothy Garton Ash warned against the erosion of free speech in Britain today. He said the public should stand up against the trend for "so-called safe spaces" and the "salami-slicing" of free speech in universities.

On the latter, he referred to the government's 'Prevent' strategy aimed at combating extremism, even of the non-violent kind. He told his audience: "Now non-violent extremists? That's Karl Marx, Rousseau, Charles Darwin, Hegel, and most clearly Jesus Christ, who was definitely a non-violent extremists. The Home Office wouldn't want him preaching on campus.

"This is a real threat I think to free speech and one we have to fight back against."

But he also warned against "no-platforming", an increasing tendency in British universities, which denies people whose views are deemed objectionable the space to argue them. He cited Germaine Greer, who faced a backlash from students at Cardiff University over her comments about transgender women (they aren't really women, she said).

Garton Ash was saying something really important. Christians need to be saying it too, but we also need to be hearing it.

In a recent lecture at the Manhattan Institute reported by the Christian Post, Jonathan Haidt, author of The Righteous Mind: Why Good People are Divided by Politics and Religion, argued that there is a "new religion" of "fundamental social justice" sweeping across American campuses.

He says causes like combating racism and homophobia have become "sacred", meaning it's difficult to have honest conversations about them.

""There is no nuance, you cannot trade off any other goods with it. So if you organise around fighting racism, fighting homophobia, fighting sexism, again all good things, but when they become sacred, when they become essentially objects of worship, fundamentalist religion, then when someone comes to class, someone comes to your campus, and they say the rape culture is exaggerated, they have committed blasphemy."

Both Haidt and Garton Ash are defenders of the right to offend, to be counter-cultural, to argue the unpopular and disturb the consensus. They are standing up against those who want to silence one side of a debate because it's assumed the argument is settled and that the other side is so dangerously obnoxious they shouldn't even be heard.

They're absolutely right, and Christians should be standing right alongside them, for two reasons.

First, denying people the right to speak openly doesn't mean they won't gain a hearing. It means their beliefs will be shared in pubs and in homes, in random conversations, on social media, among like-minded people. And because these people are like-minded, views that are wrong and dangerous won't be challenged and laughed at. They'll grow and spread, until there's a revolution.

Second, no-platforming certain views means that a government or an institution has taken it upon itself to decide what's orthodoxy and what isn't. It becomes an arbiter of truth. Germaine Greer's treatment is one example. And once that happens, there's no safety for anyone.

Now, we should just admit that Christians are in an embarrassing position here. For centuries, in many countries, we have sought to control public debate through blasphemy laws. In most historically Christian countries these are a dead letter even when they have not been explictly repealed. But now, many Christians feel that the boot is on the other foot. The new 'religion' of which Haidt speaks has become an anti-liberal sacred cause. It's not that it is opposed to religion as such, like 20th-century Communism, but it is ferociously opposed to religion that challenges its orthodoxy on matters like sex and the value of human life. Some things are becoming unsayable, or are heard only as an attack on what decent people all know to be true. Best to beware of religion; it isn't nice.

So Christians need to be very vigilant in standing up for free speech, for our own sakes. However, there are two problems to face.

One is that this can make conservative Christians in particular look distinctly unloveable, as though they are deliberately arguing for the right to be offensive. An example of this is the campaign in support of James McConnell, the Belfast pastor who was charged with making grossly offensive comments about Islam in a sermon broadcast over the internet. He was found not guilty on the grounds that his comments failed to qualify as grossly offensive. They were, however, offensive, and McConnell arguably misused his freedom of speech. To be legally free to say something doesn't mean we should feel morally free to do so.

Second: freedom is indivisible. Christians need to be as zealous in defending the freedom of others as we are in defending our own. We cannot expect to be exempt from criticism or to be able to call upon the law to defend us.

But there's a third question, too. Churches can themselves become 'safe spaces', where certain questions can't be asked and certain views aren't welcome. There is a body of orthodox Christian doctrine which is rightly to be taught and believed in every congregation. But what the Church – and society in general – has learned about free speech is that no one, in the end, loses by it. Every voice needs to be heard, every idea needs to be established, every individual's insight has value.

Follow Mark Woods on Twitter: @RevMarkWoods