Simon Kelner: For 40 days and 40 nights, fish will be my new religion

I am not religious. I am not a militant secularist. I think Richard Dawkins talks a lot of sense, but then when I hear the Archbishop of York sermonise, I find it easy to get behind him, too. I even find myself in vigorous agreement with the speaker on "Thought for the Day", wishing that I could be similarly beatific. (Although when I fail to locate my glasses and I'm already late for work, and the dog still needs walking, I discover that I'm some distance from Godliness.)

I was born Jewish, and although I don't practise, I do feel that my religion is part of who I am. Philip Roth is one of my heroes, and I like to think that I understand things in his prose that non-Jewish readers just wouldn't get. And I believe that, when Woody Allen was funny (ie before he married his daughter), he'd touch a particularly Jewish nerve. His description in Annie Hall of the moral dilemmas in a Jewish household resonates most closely with my experience: "In my family," says Alfie Singer, "the biggest sin was to buy retail."

All of which is a preamble to my declaration that, as from today, I'm going to do something I've never done before: I'm about to follow a Christian tradition and observe Lent. This, most of you will know, is the period between Ash Wednesday and Easter Sunday, and comprises 40 days and 40 nights. During this time, the modern believer commits to giving up something that may be considered a luxury as a form of penitence. Chocolate or alcohol, for instance, are most commonly on the banned list these days. Twitter was alive yesterday with alternatives, and I especially liked the woman who asked whether, as she was to give up biscuits, this would include Iced Gems, "as the base is biscuity".

I, however, am going back to a more traditional era, a time when all animal products were strictly forbidden. I am not quite as hardline as the Catholic priest and philosopher Thomas Aquinas, who said in the 13th century that meat and dairy products "afford greater pleasure as food [than fish] and greater nourishment to the human body, so that from their consumption there results a greater surplus available for seminal matter, which when abundant becomes a great incentive to lust".

I don't necessarily buy that, but I have made a pledge: I am giving up meat for Lent. I have long been attracted by the argument that meat production causes massive environmental degradation, and many dietary experts say that eating only fish and vegetables will bring considerable health benefits. So while the framework for my period of abstinence comes from religious doctrine, the incentive for my own particular fast has a more practical grounding.

I have my first test at lunchtime when I'm being taken to one of London's most renowned steak houses. A sirloin, sir? Maybe a piece of grilled fish instead. No, I'm not religious. But for the next 40 days I might as well be.