Theresa May has given up crisps for Lent. A small thing, you may say, when compared with giving up the European Union, for instance. And did she know that the alt-right have been boycotting Walker’s crisps because of Gary Lineker’s support for refugees?

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Probably not – May was doing the traditional Lenten thing of giving things up, after the traditional Christian practice of fasting. But in our topsy-turvy society where rich people aspire to be pencil-thin and poor people are abused for being fat, giving up crisps or chocolate is usually more about aesthetic self-regard than it is about reminding yourself of your existential fragility. Moreover, in an age when austerity is ideologically imposed on the poor, the idea of giving up crisps as being something virtuous cooperates with the rightwing philosophy that austerity is morally advantageous for the poor themselves.

Tonight, at the Ash Wednesday service, I will tell all my congregation that they are going to die. One on one, I will mark them with ash and whisper in their ear: “Know that you are dust and to dust you shall return.” With this knowledge, the season of Lent begins. But whereas the raw evidence for this knowledge used to surround us, these days most people die in private or in hospital, surrounded by machines rather than their families. The vast majority of young people have never seen a dead body. And when the Radio 4 presenter Steve Hewlett spoke openly about his forthcoming death, it became quite a thing. But how odd that speaking about something that all of us have in common should become a matter of such note. The Victorians used to talk openly about death and were weird about sex. We talk openly about sex and are weird about death. And that’s because what death says is that you are not the centre of the universe. It absolutely doesn’t say: “Because you’re worth it.”

The prophet Isaiah had some pretty harsh things to say about people who misunderstood the purpose of fasting. “Look, you serve your own interest on your fast day and oppress all your workers,” said the prophet. “Is not this the fast that I choose: to loose the bonds of injustice, to undo the thongs of the yoke, to let the oppressed go free, and to break every yoke? Is it not to share your bread with the hungry, and bring the homeless poor into your house; when you see the naked, to cover them?” In other words, how can May give up crisps for Lent while excluding the Dubs children? Isaiah would have exploded with rage.

The reason Christians fast in Lent is not to make them thinner or more beautiful. It’s to bring them closer to their own mortality. And to hear more clearly, and thus to view more suspiciously, the insistent yells of the beast desire, banging away in one’s breast, always wanting more, and always wanting it to be all about me.

Lent is a 40-day course in other-centredness – seeking to put others before oneself. And 40 days because this was the number of years the Jewish people wandered in the wilderness as refugees. Fasting is not about getting into a size 10 dress. It’s about recognising our own fundamental vulnerabilities, and being attentive to the vulnerabilities of others, especially those with nothing. So my friendly, priestly, pastoral advice to the prime minister would be to forget about the crisps. Doing without your regular salt-and-vinegar fix won’t really bring you closer to God. But changing your mind about the Dubs refugees really will.