Is Simon Cowell about to convert to Islam? The family of his fiancée, Mezhgan Hussainy, apparently wants the billionaire music mogul to become a Muslim if the couple are to marry. "They're very westernised, but no one in their family has ever married a non-Muslim - and they're not willing for their youngest daughter to become the first," a "source" is reported to have said of Hussainy's Afghan-born parents.

So will Simon become Suleiman? "Don't convert to Islam, Simon. Don't do it," says the right-wing US pundit Debbie Schlussel on her blog. For once, I can't help but agree. What do Muslims gain from the conversion of Cowell? New tunes? It frustrates me how my co-religionists get so excited at the prospect of people converting to Islam, especially celebrities. Last year, I was inundated with emails claiming Michael Jackson had been on the verge of a deathbed conversion to Islam. Heaven forbid.

“Islam is the fastest-growing religion on earth," Muslim friends say with glee. Indeed, but I don't know how adding to the world's Muslim population helps reduce the theological, political, cultural and socio-economic problems blighting its communities. I sympathise with the senior Indian Muslim cleric who revealed to me his (private) advice to Hindus considering converting: "Don't bother. Not until we Muslims get our house in order."

One-way ticket

Then there are those who convert to Islam only to bring it into disrepute. They shouldn't bother. Take Colleen LaRose, aka Jihad Jane, the 46-year-old blonde American convert from Pennsylvania who was accused in March of plotting to murder a Swedish cartoonist. She is the latest in a long line of western converts to radical Islam, including John Walker Lindh ("American Talib"), Richard Reid ("Shoe Bomber") and José Padilla ("Dirty Bomber").

However, the bigger issue that many Muslims choose to ignore is how Islam has become a one-way street. Non-Muslims are encouraged to convert to Islam, but Muslims are forbidden from converting away from Islam. The sharia (or Islamic law), it is claimed, sanctions the death penalty for any adult Muslim who chooses to leave the faith, or apostatise. This is an intellectually, morally and, perhaps above all, theologically unsustainable position.

Intellectually, it makes no sense to seek converts to Islam while laws backed by capital punishment curtail conversions the other way. In Kuwait in 1996, amid a row over the conversion of a businessman to Christianity, one cleric argued: "We always remind those who want to convert to Islam that they enter through a door but that there is no way out." Yet what of those who were born Muslims? And how do you stop people changing their minds?

Morally, communities throughout the world believe that human dignity depends on freedom of thought and conscience, as enshrined in the Universal Declaration of Human Rights, ratified in 1948. This right, states Article 18, "includes freedom to change his religion or belief". Muslims often forget that, while the puritanical Saudi government opposed the document, the bulk of Muslim-majority nations signed up. The then Pakistani foreign minister, Muhammad Zafarullah Khan, wrote: "Belief is a matter of conscience, and conscience cannot be compelled." Yet various Middle Eastern states, including Saudi Arabia, Iran and Sudan, retain the death penalty for apostates, and at least 14 Muslim-majority countries make conversion away from Islam illegal.

Theologically, however, there is no justification for such punishment. As India's leading Muslim reformist scholar Asghar Ali Engineer has pointed out: "If some ulama [Islamic jurists] insist on death sentence for apostasy, it is not only a crime against freedom of conscience and democratic rights, but also a serious disservice against Islam." I, for one, refuse to worship a God who is so weak and needy that he compels Muslims to worship him.

Nowhere in the Quran is it suggested that a Muslim who renounces the faith should be given the death penalty - or any penalty. Indeed, Islam's holy book proclaims, "Let there be no compulsion in religion", and elsewhere that "Those who believe then disbelieve, again believe and again disbelieve, then increase in disbelief, Allah will never forgive them nor guide them to the Way." This verse suggests only a heavenly punishment, not a worldly one.

Freedom fighters

The claim that apostasy is a capital offence results from medieval Muslim jurists' conflation of apostasy with treason. Treason remains a capital offence in some secular, western countries, including the US, as it does within Islamic law. But the Prophet Muhammad never had anyone executed for apostasy alone. In one case, in which a Bedouin man cancelled his pledge of allegiance to Islam and left Medina, the Prophet only remarked that "Medina is like a pair of bellows: it expels its impurities and brightens and clears its good."

Muslim reformers are often delighted to discover that the foundations for religious freedom lie in their own faith traditions. And the fight for religious freedom in Muslim societies goes hand in hand with the fight for political freedom. How can Muslims be expected to exercise freedom of speech or assembly, if they cannot be guaranteed freedom of thought and conscience? There are grounds for optimism.

“Even in conservative societies, Muslims are beginning to realise that faith is a matter of personal responsibility and not a consequence of authoritarian decree," wrote the US physicist and Muslim reformist Hasan Zillur Rahim in 2006. "The days of religious leaders thundering: 'I am right, you are dead' will soon, let us pray, be over once and for all."