What is the Protest the Pope movement really about? Ask ten different activists why they object to Mr Joseph Ratzinger's state visit to Britain and they'll give you ten different answers. My own mother, a lapsed Catholic who is not normally the sort of lady to spoil her shoes on noisy demonstrations, looked me in the eye when I inquired about her reasons for attending the protest and said, very quietly, "it's personal".

For many of the millions who are dismayed at the prospect of the papal visit, it is indeed personal, and it is also political. Deep-rooted resentment drawn from first or second-hand experience of the institutional brutalities of Britain's largest "minority" faith - there are nine million Catholics in this country- forms the basis of legitimate liberal indignation.

It is hard to pinpoint exactly what offends most about Ratzinger's visit: it his attempts to rehabilitate child-rape within the church, or his intolerant stance on safe sex and abortion? Is it his relentless persecution of homosexuals, or the fact that public funds are being spent on driving this horrible old man around the country in his shiny white popemobile? It is all of these things, and none.

It is about religion. It is about what organised religion does to human societies. It is about the British and our unique distaste for being told what to do by celibates in silly cassocks. It is about modernity, and the limits of what modernity will tolerate.

Anarchist journalist Angelo Quattrocchi wrote in his recently-released polemic "The Pope is Not Gay!" that Ratzinger's backward-looking moral crusade is "spitting into the wind" - expectorating intolerant bile that can only cause unnecessary suffering as a tremendous tsunami of modern tolerance surges forward to swamp the rotten structures of family, patriarchy, superstition and sexual prudery. The Protest the Pope movement is, in fact, so terribly modern that it can sometimes come across as a little smug.

This week's Southbank launch of Quattrochi's book, replete with heartfelt performance poetry and expensive box-wine, was a restrained orgy of liberal self-congratulation. But why on earth shouldn't we congratulate ourselves? We are one of the most tolerant cultures on the planet, taking a stand, in the midst of domestic turmoil, against global religious oppression. Can't we feel just a little bit proud?

Most of those planning to 'Protest the Pope' this weekend are not instinctively anti-Catholic; we have no issue with belief itself. The notion of taking special exception to one religion over and above any other dodgy cult cobbled together by deranged desert patriarchs should be abhorrent to any secularist who believes in freedom of thought. When Elizabeth I granted private amnesty to English Catholics, she declared that she had no desire to "make windows into men's souls", and nor do today's Pope Protestors. We simply wish to register our displeasure when the same believers dash around smashing in the animistic vitrines of their fellow citizens with big bricks made of bigotry and intolerance.

A few short weeks ago, senior priest Edmund Adamus condemned Britain in general and London in particular as a "hedonistic wasteland" - the "geopolitical epicentre of the culture of death' - because of our dirty little fetish for protecting homosexuals from discrimination, supporting single parents and preserving limited access to abortion services.

If believing in a woman's right to choose is anti-Catholic, then I am an anti-Catholic. If believing that homosexuals deserve absolute legal and social equality with heterosexuals is anti-Catholic, then Britain is full of anti-Catholics.

If it is anti-Catholic to believe that child-rape ought to be eliminated, that stopping the spread of AIDs in Africa trumps religious squeamishness about condom use, and that human happiness is more important than dogmatic adherence to cobweb-crusted notions of purity and morality, then I for one am proud to be part of the geopolitical culture of death.

On Saturday, I'll be marching through my home city beside thousands of others to tell bigots and dogmatists everywhere that if they try to push back at the raw edge of modernity, they're going to get cut. If that conviction makes me anti-Catholic, then just give me a pen and show me where to sign.

Read Laurie Penny's column every week in the New Statesman.