The High Court has ruled that Christian evangelists can't bring a prosecution for blasphemy against Mark Thompson, the head of the BBC, for his decision to air Jerry Springer — the Opera. This was a commendable decision.

The right to say offensive things about other people’s ideas is one of the mainstays of democracy. Why should the law give religious ideas special protection from criticism just because the holders of those ideas say it is a mortal sin to offend them?

Blasphemy is a crime in England, but it has become virtually defunct. The offence is defined as the publication of “contemptuous, reviling, scurrilous or ludicrous matter” relating only to God, Jesus Christ, the bible or the beliefs of the Church of England — it is not blasphemy to attack any religion other than Christianity.

In 1989, the film Visions of Ecstasy, directed by Nigel Wingrove, was banned in Britain for blasphemy because of its depictions of the erotic fantasies of St. Theresa of Avila, a 16th-century Spanish Carmelite nun who asserted that she had ''raptures'' about Christ. On appeal, the European Court of Human Rights ruled that a state can have blasphemy laws because they are justified exceptions to freedom of expression.

For the most part, however, our law has evolved to become tolerant of people critical of Christianity. Prosecutions for blasphemy are extremely rare.

In 2006, Parliament passed the Racial and Religious Hatred Act, which makes it illegal to stir up hatred against people on the basis of their religion, but in section 29J it explicitly permits criticism of any set of religious ideas.

This is a wise law. People can’t change their race or the colour of their skin, so hating them for it is irrational. But your ideas are something that should be open to criticism. No group should be given special protection against being insulted or offended if its ideas are attacked. There’s a world of difference between saying “anyone who believes the earth is flat is being stupid” (an attack on a belief) and “anyone who is English is stupid” (an attack on people).

The new offence of stirring up hatred against people on religious grounds covers words, behaviour and written and broadcast material; to be an offence, that material must be threatening and intended to stir up religious hatred. The victims must be defined by their religious belief or lack of it (though the Act doesn't state what, exactly, constitutes a religion or religious belief: that is left to the courts to determine.)

After a lively Parliamentary debate, section 29J was inserted into the 2006 Act in an attempt to protect fee speech. It states that nothing in the Act should be interpreted in any way that would restrict debate or prevent criticism of particular religious beliefs.

Imagine the consequences if that section hadn’t been inserted. A religion that, for instance, condoned incest or forbade children from learning science would be able to preach such practices with impunity. Anyone trying to speak against it would be punished for insulting the believers.

We must allow people to hear all sides of an argument about beliefs so that they can make up their own minds. How good can a set of ideas be if the only way its believers can keep them alive is to ensure that it is illegal to criticise them? As the American judge Oliver Wendell Holmes, speaking about conflicting ideas in a case in 1916, said: “The best test of truth is the power of the thought to get itself accepted in the competition of the market”.

Those who say want to punish others for criticising their ideas clearly can’t, deep down, have much faith in the merits of their ideas.

Professor Gary Slapper is Director of the Centre for Law at The Open University

Times Recommends

• The best legal yarns this Christmas
• Legal school 'earned £1m by exploiting bar course'
• Axe may fall on 5,000 more City lawyers