The bestselling writer and environmentalist says that living sustainably is a vital part of Christian life and worship

There are of course some Catholics noisily unconvinced of his merits, but for climate-change campaigners of all faiths and none there is no question mark over the hero status of Pope Francis. “Laudato Si’ was a clarion call for the world to wake up and get engaged,” says the theologian and environment activist, Ruth Valerio.

“Having someone of his stature giving voice to this has enabled others to speak out. His phrase ‘our common home’ was genius: it’s a beautiful way of expressing that we share the world with all of humanity. Because if this is our common home, we should behave differently. You wouldn’t have hungry people in your house while you were eating a big meal round the dining-room table.”

This Lent, Valerio is putting climate change and ecological concerns into sharper Christian focus. Her latest book, Saying Yes to Life, is currently flying – or maybe that should be taking the fast train – off the bookshop shelves. So the first question, when we meet for tea at a central London cafe, is: what difference does it actually make, ecologically speaking, whether you’re a believer or not? It’s the world, after all, that we’re saving, and we all live in it, whatever our religious beliefs.