The Opus Dei Enigma
It's widely regarded as one of the most secretive religious organisations in the world. It makes heavy demands on its members - and has been accused of cult-like practices. It's also an influential movement within Roman Catholicism. Opus Dei, made famous by Dan Brown's bestselling novel the Da Vinci Code, has many critics - but few have found out what life is like on the inside.
The BBC has been granted exclusive access to the movement's extensive headquarters in Rome - which include a university, a centre for groundbreaking biomedical research, and a seminary where priests are trained. Why does it wield such power? And is that on the increase under Pope Benedict?
The BBC's religious affairs correspondent Christopher Landau meets both priests and lay people who devote their lives (and their money) to this movement which, though less than 100 years old, exerts powerful influence over both its members and the wider church.