As I believe in freedom of opinion – as well as God – I have no problem with London's buses carrying the slogan "There is probably no God"; although I would admire the bravery of the advertisers more if they added "or Allah". But, if people want to contribute £100,000 to an advertising slogan – great. It shows that capitalism (and its adjunct, marketing) works in the cause of freedom of opinion. The best guarantee of freedom of expression is not a charter of rights or a bill of free speech, but diversity in media and communications.

I don't agree with the slogan in question, but then I don't agree with a lot of advertised opinions. I know them to be bunkum, like the beauty creams that offer to "turn back time". I disbelieve such claims because they do not accord with my experience: such lotions have been employed for years, and time has marched relentlessly on. The coda in the London bus secularist message "so relax and enjoy life" strikes me as bunkum rather on a par with beauty cream claims, and for the same reasons: it does not accord with my experience.

Far from relaxing and enjoying life, most atheists I have encountered are gloomy blighters with a depressing and nihilistic message that there is no purpose to life so where's the point of anything? They so often fall into the category defined by GK Chesterton: "Those that do not have the faith/Will not have the fun." You only have to attend one of their dreary humanist funerals to see that – I am never going to another of those, just to be made miserable.

I'm not against people having different opinions; what I'm against is people in influential positions never having their opinions challenged. Far more nauseating than the open slogans on the London buses, subscribed to and paid for, are such phenomena as art installations by the likes of Gilbert and George consisting of the words "Ban religion", over and over again. They are then fawningly interviewed by a deferential Evan Davis on BBC Radio 4, when they should be subjected to the same kind of rigour as John Humphrys doing over the chancellor of the exchequer. Incidentally, if Gilbert and George had lived in a country which banned religion – Enver Hoxha's Albania – they would have been dispatched to perform menial agricultural work, if not to a gulag.

In short, let us have discourse and debate about these matters of faith and non-faith. Does it not say in St Paul: "Come, let us reason together"? Advertising is discourse: it is a form of communication which people are free to accept or reject. It probably further stimulates debate. It may even stimulate someone to write a book called "The Tolerant Atheist", in which the author affirms that while he is not a believer himself, he accepts the evidence that for countless numbers, religious faith gets them through the night: and its beauty, language, optimism, music and architecture helps them relax and enjoy life.