An unassuming man slipped into Scotland at the end of last week. No paparazzi met him off the plane. After all, he has not been on Big Brother, he has not supped milk, dressed in a leotard, alongside George Galloway; nor has he been on the front page of Hello! magazine. His only claim to fame - though he is not famous - is that he is one of the spiritual giants of our time.

Jean Vanier is a man who gives Christianity a good name. He came to Scotland to speak in the Usher Hall at the invitation of the organisers of the 2007 Festival of Middle Eastern Spirituality and Peace, a group which grew out of a small conference of Jewish and Muslim peacemakers. But why invite Jean Vanier? To answer that question it's necessary to understand something of the exceptional journey this man has been on. Son of a Canadian diplomat who was to become Governor General of Canada, Jean was profoundly moved, at the age of 17, when he accompanied his mother to meet starving Holocaust survivors. The impressionable youth was shocked by the sight of the skeletal figures, and by the evidence of the cruel things human beings could do to each other. By this time Vanier had decided to make a career in the navy. In his early 20s, responding to heart-felt promptings, he decided to leave the navy and study philosophy in Paris. After receiving his doctorate, he taught philosophy in Toronto.

It was a visit to his spiritual mentor, Father Thomas Philippe, chaplain at an institution for people with developmental disabilities, that changed his life. Jean was disturbed to see the dreadful conditions in which the inmates were living - locked away from the rest of society, and leading dismal, unproductive lives. Vanier chose to follow his heart and took a risk. In 1964 he bought a small house in the French village of Trosly-Breuil and invited two men from an institution to share it with him. He called the house "L'Arche", after Noah's Ark. Soon, young people came to share life in this new kind of community. Vanier knew that they would find what he had discovered: that the gifts of the physically and mentally disabled are as precious and enriching as any help given by the willing assistants. Today, there are 130 L'Arche communities in 30 countries on five continents (with one house in Inverness and another in Edinburgh).

Jean Vanier, then, has made a difference in the lives of countless people around the world - those with disabilities, their families and the many young people who have chosen to help in L'Arche communities. But there's more to him. In the course of his personal pilgrimage, this Catholic layman has developed a profound spirituality which makes him one of the most loved and admired figures in the ecumenical movement today. Seven years ago, Vanier was invited to address the Kirk's general assembly. None of us in that hall will ever forget the sheer power of the message of the quiet-spoken man who still shares his day-to-day life with people who have learning difficulties. He taught us how people in need - in a society which excludes them - want not just generosity but relationship, communion; and how, as human beings, we all learn to hide our inner brokenness and vulnerability behind the impenetrable screens of our capacities and our power. As he told Colin Mackay yesterday on BBC Radio Scotland: "To become profoundly human is not to have power but to grow in love."

Jean Vanier's God is a vulnerable deity, one who slips incognito into the margins of human life, where people bleed and weep. In his life experience and in his reflections, he expresses a religion of the heart, one which embodies the theology of the second-century theologian Irenaeus: "The glory of God is a human being fully alive."

Compare that, if you will, with the grotesque kick-ass, stereotyping philosophy of the religious right, engaging in a shock-and-awe, anti-Islam crusade on behalf of a God who is a signed-up Christian warrior.

Vanier is an irreverently joyful prophet for our time. (He will never, of course, be interviewed on television by Richard Dawkins, whose idea of objectivity is to focus all his attention on religious freak shows.) He offers an alternative vision of how to be religious. His L'Arche communities in India, with their Hindu, Christian, Muslim and non-religious assistants, are fragile, hope-bearing signs of compassionate and respectful living. And his presence in Scotland - he is also visiting Saughton Prison to meet some prisoners who have been reading one of his books - reminds us of the point made by the great Welsh poet RS Thomas: It is too late to start For destinations not of the heart.