There are few things quite so funny as a lugubrious preacher talking about joy. Rikki Fulton made a highly successful second career out of it. The more his famous character, the Reverend I M Jolly, talked about Christian joy, the sadder, and funnier, he became. It was said of one real-life hangdog preacher that he often spoke about joy, "but he forgot to tell his face".

I thought about this when I read that members of the Free Church of Scotland have been urged to lighten up by the Rev David Robertson, editor of the church's Monthly Record. Referring to Rikki's IM Jolly and W E Free, he wrote: "They are caricatures but sadly they are popular because they reflect a common understanding. Religion, especially of the Scottish Presbyterian kind, is doom, gloom, blackness, depressive and joyless.

Let congregations pray, think and act in order to deal with the sin of joyless worship."

Robertson, Free Kirk minister in Dundee, is one of the sharpest tools in the conservative evangelical box. A thinker who seeks to understand modern cultural trends and to engage with them from a theological perspective, he wants the Free Kirk to move out of a cultural ghetto. He is certainly right to draw attention to the dour image of Scottish Presbyterianism, particularly in its narrower manifestations.

Why the gloom? I think there's a book to be written about the relationship between geography, weather and theology. Not surprisingly, Puritanism seems to thrive in northern climes; fending off the pestilence of midges in the summer followed by leaning into gales in perma-dark chilling winter days is hardly likely to engender a sunny disposition. It's not easy to "hang loose" when you're wearing three duffle coats. There will be no dancing in the western aisles.

As someone who belongs to the decaff version of Scottish Presbyterianism - the Church of Scotland - rather than the full-on, heart-racing blend that can kill you, I have a sneaking regard for the wilder Presbyterian reaches. There are many good people there. I rather like the melancholy, spine-tingling edge of the Gaelic psalms, and would hate to see this rugged tradition replaced by banal happy-clappy choruses. I also have an aversion to perpetually cheery hand-waving Christians with welded-on smiles. In fact, they make me rather ill. When you throw in tele-evangelists with gleaming teeth and cream suits, I'm searching frantically for the exits. Give me the gritty integrity of the Western Isles psalm singers any day.

The biggest problem with the nether reaches of Scottish Presbyterianism is a predilection for breaking away from the main group of punters in the name of an idealised and unattainable holiness. Institutionally, the self-obsessed schismatic Presbyterian bumblebee ends up flying up its own backside. Much worse is its fatal attraction to fratricide. Every so often the Taliban tendency demands a human sacrifice. This is dark and dangerous stuff, and no cheery choruses will wash the blood away.

All of this raises an intriguing question, for politicians as well as religious leaders: at what point does behaviour totally undermine professed beliefs? This is less straightforward than it seems. One of my favourite poets is R S Thomas, the cantankerous Welsh clergyman who could never be mistaken for a ray of sunshine. Yet this glowering priest is rightly regarded as one of the finest poets of the twentieth century. Welsh ogre he might have been, but his poetry is sublime. The truth is there, both despite, and because of, the life.

Chogyam Trungpa, co-founder of the Samye Ling Tibetan monastery in Scotland, was one of the great Buddhist teachers of modern times. Yet no serene ascetic was he. He would give his teachings reeking of alcohol, and his relationships with some of his female students were less than saintly. He walked with a limp, the result of a car accident in which the Tibetan guru drove through the window of a joke-shop in Dumfries. Yet his books have become modern spiritual classics. That's life, as a Buddhist might say with a shrug. The point at which people's public theses are invalidated by their private failings is a tricky one to pin down.

Christianity has always urged adherents to live up to the best of the faith, yet recognises the inevitability of failure and the need for forgiveness. The most merciless witch-hunters today are actually those pure creatures, tabloid editors. The slightest inconsistency between life and letters is swooped upon; even football managers nowadays have to lead lives of pristine purity.

Back at the Free Kirk ranch: David Robertson is right to hint that some of the fathers and brethren should attend "The Smiling School for Calvinists", to quote the memorable title of Bill Duncan's book. Mind you, Robertson isn't asking for chirpy-chirpy-cheap-cheap Christianity. He wants what he calls "serious joy". I'd settle for that.