By Christopher Landau
BBC Religious Affairs Correspondent
The Opus Dei headquarters is in the underground church in Villa Tevere
If ever a religious movement has been defined in the public imagination by its depiction in a novel, it is the Catholic group Opus Dei.
Dan Browns bestseller The Da Vinci Code portrayed a mysterious, cult-like organisation, shrouded from the wider world.
Opus Dei insists the reality is very different and I was given a rare chance to see the wide variety of initiatives they undertake in Rome.
Some aspects, if not mysterious, are certainly intriguing. For example, Opus Dei tends to be extremely discreet about the function of its buildings.
The groups large seminary where priests are trained in central Rome is unidentifiable from the outside.
One small doorway in a high wall is apparently the only entrance, even though the community is home to more than 100 men.
Conspiracy theorists might start to raise their eyebrows but inside, I found a uniformly warm welcome from students insistent that there was nothing odd about being trained by Opus Dei.
Follow the rules
Members of Opus Dei have to attend church every day
Singing their evening prayers in a chapel built long before Opus Dei took over the ancient building, the seminarians are drawn from countries around the world.
They speak of the honour of being sent to Rome to be trained by the group, and clearly have high regard for what they refer to as the holiness of the priests they learn from.
However, it is ordinary Catholic lay people, rather than priests, who have dominated the membership of Opus Dei, ever since it was founded in 1928 by the Spanish priest Josemaria Escriva, who was later made a saint.
People like Marta Brancatisano, who is not a mysterious, shadowy religious figure, but a middle-aged mother and lecturer at Opus Deis university in Rome.
Marta teaches about family life, and the Catholic values that should underpin it, at the Pontifical University of the Holy Cross.
The name itself is significant. To be honoured by the Vatican as a pontifical university shows that Opus Dei is regarded by the Church as part of Romes intellectual Catholic establishment.
I met Marta just after she had finished a lecture, which included comments about the dangers of social networking sites undermining traditional family values.
She could hardly be more effusive in her praise for Opus Dei, regarding it as a way to see beyond the troubles of everyday life. This is freedom, this is joy. If it is real, this is paradise on Earth," she said. Opus Dei makes strict demands on its members, as it helps them find what is referred to as holiness in everyday life.
Each day they are expected to attend church and make time for personal prayer and devotion.
Most members of Opus Dei are married. But single members, called numeraries by the movement, are also asked to undertake a spiritual discipline known as corporal mortification.
It involves self-inflicting pain in order to focus on Christs suffering on the Cross, something also practised by some other Catholics.
Opus Dei is also concerned with the relief of suffering, and its innovation in healthcare is perhaps the most remarkable and unexpected aspect of the groups work.
They want to find medical treatments that conform to Catholic values
On a large site on the outskirts of the city, a massive hospital and biomedical research centre is Opus Dei's latest initiative.
Members of the public are treated free of charge and research laboratories work to find medical treatments that conform to Catholic values.
Rome is also the headquarters for the groups new overseas development charity, Harambee, providing microfinance to African communities.
One man who is candid about opposition within the wider church to Opus Deis work is Sean Patrick Lovett, a senior official at Vatican Radio.
Theres jealousy, theres envy, theres confusion, and I think theyre working very hard to clarify that and help people to understand who they are, what they do and why they do it.
It may have become famous through the thrilling fiction of Dan Brown but the reality of Opus Dei is certainly less dramatic, perhaps even a bit more mundane.
Opus Dei should not be underestimated. While relatively small in numbers, it has found ways to extract lives of committed service from its members.
The movement now plans international expansion, into new countries like Indonesia, where it does not yet have a presence.
As it spreads its message of finding holiness in everyday life, it seems determined to continue growing its influence.
Assignment is broadcast on BBC World Service on Thursday at 0906 GMT and repeated at 1406 GMT, 1906 GMT, 2306 GMT and on Saturday at 1106 GMT.
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