FORGIVE me: I'm in a bad mood. I'm not the sort of priest who rams religion down anyone's throat - or indeed who tries to insert it through any other orifice apart from the ear.

So I don't expect half the yuppies in the City of London to turn up to my brief weekday lunchtime services of Holy Communion. But I do expect those who do turn up to know their manners.

I'm not talking about your actual worshippers, but about another variety of the human species altogether. These people arrive just after the service has started and they stand half way to the back watching the rest of us say our prayers - as if they were anthropologists observing a strange and exotic tribe of savages.

Not content merely to peer at us in a respectful silence, they cough and gossip and rustle their plastic bags - the plastic bags being the outward and visible signs of their own religion, which is of course shopping. Our church is vast and cavernous, the largest church in the City of London. And so it has a hell - so to speak - of an echo. The result is that neither priest nor congregation can hear themselves acknowledge and bewail their manifold sins and wickedness.

Most people who enter a church behave reverently, but there are always a few oiks - often most affluent and smartly-turned out, but oiks nonetheless - who don't. I recall wandering into church one Saturday morning before a wedding and finding the best man noisily shifting furniture in the sanctuary. An electric plug on the end of a cable in his hand, he grunted: "Where do I put this?" I confess I told him exactly where to put it.

The mobile phone - that trademark of the ubiquitous idiot - is a blight on religious observance. They go off during weddings, funerals, City memorial services - you name the religious occasion and I'll guarantee you an outbreak of babyish ring-tones to ruin it. What wonderful ecclesiastical version should we expect of "I'm on a train!" What else might we hear from these pestilential, self-obsessed nuisances? "I'm saying my prayers - can I ring you back?" Or: "You have reached the voice-mail of Candida Tattoo-Nosestud. I'm sorry I can't answer the phone at the moment: I'm making my confession."

The most blatantly horrific violation of religious space happened at a Thursday lunchtime said Mass. A young lady - well, I am trying to be polite - in what I can only describe as a gownless evening strap was swivelling her neck through 360 degrees throughout the service and showing her teeth to any who might be interested. I was chiding myself for being irritated. "Come on Mullen, you ought to be more charitable. That lady is a child of God after all." But then she came up to the altar for Communion.

As she knelt there, her mobile went off. You might imagine she would have fainted with embarrassment. Not at all. Without a flicker of hesitation, she held the mobile to her ear and answered it, while holding out her other hand to receive the Blessed Sacrament. It reminded me of a Royal garden party at which someone was being presented to the Queen. The guest's mobile went off. Her Majesty, unflustered as ever, said: "Oh do get that: it's bound to be someone important."

* Peter Mullen is Rector of St Michael's, Cornhill, in the City of London, and Chaplain to the Stock Exchange.