Causes can become fashionable. Care for the environment is one obvious example. All kinds of people are taking an interest, many of them ordinary, but some are celebrities. Support from celebrities can be invaluable. It can help to promote a worthy cause. But then celebrities' motives are also scrutinised: are they truly committed to the cause, people ask, or are they acting out of self-interest, to raise their own profile and popularity? Last month the Los Angeles Times noted: “Brad Pitt and Angelina Jolie can adopt three children from impoverished nations, travel the world to promote humanitarian aid, and still have to answer for a helicopter ride ...from Manhattan to a Hamptons' fundraiser for Pitt's green-home-building project in New Orleans.” And then it reported that “to be truly green, a celebrity must not just carbon-offset or advocate for the cause, they must relinquish things”, such as the private jet and multiple homes and garages filled with cars.

At another level the challenge goes even farther. It is not only inquiring whether a person's lifestyle is in tune with what they say; it is also asking about sacrifice. Care for the environment is costly. When people champion this cause, but can shield themselves from its consequences, they fall under suspicion. Of course, the matter is not straightforward. A celebrity's presence may boost the impact of a significant event. We need to be alert to the question, “Does one cross-country ride in a private jet cancel out the vegetarianism and the bamboo floors?” But, when a cause's consequences make demands, talk without sacrifice raises suspicions.

Green issues are one example, but they throw light on our actions more generally. Someone said to me recently: “The Gospel is right. When a wealthy person gives generously to charity, he has normally given about half of what he can afford.”

The point is illustrated partly by the parable to be heard in churches this weekend, in which the Pharisee and the tax collector go up to the temple to pray. The Pharisee is full of himself, proclaiming that he is not grasping, unjust and adulterous, and certainly not like the tax collector, for he fasts and pays tithes on all he gets, while the tax collector stays at a distance and acknowledges his sinfulness (Luke xviii, 9-14). And it is illustrated as well by that other incident when the rich were putting large sums into the treasury, while a poor widow put in two copper coins. She is praised for contributing more than the wealthy, for they gave from their abundance and she from her poverty (Mark xii, 41-4). When the wealthy give, it is indeed normally half of what they can afford.

The Pharisee is unattractive because he is arrogant, but we should not rush to judge him. Perhaps he also had many calls on his wealth, many responsibilities to fulfil. All the same, his self-satisfaction grates. When Mother Teresa used to say “Give till it hurts”, she wasn't encouraging destitution, but challenging us to give that little bit more, because true generosity begins when we start to feel stretched.

So this Gospel theme is not a pretext for despising the Pharisee who is smug, or for mocking celebrities who cash in on fashionable causes. It is rather an opportunity toexamine ourselves. The parable has a cutting edge.

Many of us earn far more than we need to survive. We may support causes and give generously to those in need, but do we ever feel the pinch? Is our charity always calculated so that we are never at risk? The philosopher Anthony Kenny has described “the average selfish man” as someone who spends far more on his own superfluous comforts than on aid to those in extreme poverty, and has confessed that he compares poorly with this average selfish man.

How many of us could avoid making that same confession?

Monsignor Roderick Strange is the Rector of the Pontifical Beda College, Rome