The United States, Turkey and India all have secular constitutions. However, none of them contains the definitive "secularism". There is no such thing.

But these constitutions have a common aim: to protect religion. By not permitting the establishment of any particular faith, secularism seeks to ensure that the state cannot be used as an instrument to persecute minority religious communities, and that no religion can be imposed by law on an unwilling populace.

Secularism works both ways - while it does not permit religion to interfere in affairs of state, it also forbids the state from seeking to control religion. This is surely the best way to ensure that those with faith can pursue their beliefs unencumbered.

Equally, it permits all people to be secure in the knowledge that no outside authority will interfere with their search for meaning. Naturally, clashes will occur when religious conscience seeks to place itself above the law - for example, in granting "conscience" opt-outs for pharmacists. These conflicts are difficult, but they are not beyond resolution.

As can be seen in the US, secularism, far from challenging belief, can in fact nurture and encourage faith in all its forms and approaches. Yet it does not always work. In all the countries mentioned above, secularism is under pressure from resurgent and politicised religion. In Turkey, Islamists seek to reverse the reforms of Atatürk; in the US, Christian evangelicals work tirelessly to undermine the church-state divide; in India, Hindu nationalists kill and maim in their campaign to overturn the country's constitutional protections for Muslim and Christian minorities. Elsewhere, Christianity is again politically resurgent - flying on the coat-tails of an expansionist Islam.

Secularism is far from negative. When it opposes "faith schools", it makes the case for inclusive education; when it opposes bishops in the House of Lords, it is supporting a purer democracy; when it opposes religious opt-outs from anti-discrimination legislation, it is protecting the rights of minorities to live free from the diktats of religious bigotry; when it opposes blasphemy, it defends free speech; when it speaks out against sharia law, it promotes one law for all.

Coupled with a strong democratic structure, secular societies protect us all from the authoritarianism that is characteristic of religion when it has temporal power. Faith groups regard evangelisation as their divine duty and, given the opportunity, they will pursue it using force and coercion.

As the many religions that thrive in Britain today grow more assertive and demanding, secularism may ultimately become the only way to thwart their power-seeking ambition. We need to recognise that sooner rather than later.

Terry Sanderson is president of the National Secular Society.