I have more knowledge of it than I would like, having studied at the Opus Dei-owned University of Navarre in Pamplona, northern Spain, in the early 1990s when it was the top university to go to.
On beautiful spring days, in-love couples would keep a distance (Opus Dei condemns pre-marital sex).
Students dressed conservatively. The system favoured those close to what we called "the cause", who dominated the clubs or extracurricular groups.
This crowd would also lead tutorials, leaving the contrarians at the back (I was an independent minority, sitting towards the front). Some students came from Opus Dei-owned schools and knew how to play the system, even if they didn't always follow strict Opus Dei guidance.
Others were full members: rich (it is a private university), smart, attractive 20-year-olds who avoided the opposite sex in order to avoid temptation. I had friends among both groups. Some were helped by a celice, a spiked ring, worn so tightly around their legs that they would bleed, in order to suppress desire.
Opus Dei membership, in theory, is not allowed until 18, but I knew people who had opted for celibacy at 13, and had spent years using the celice. I have stayed in touch with some of my fellow students. A few have now left the organisation, in some cases rather traumatically.
Not all Opus Dei members are celibate. The group also includes "supernumeraries", members who marry and who follow strict guidelines such as daily mass attendance or the Catholic church's ban on contraception.
Needless to say, Opus Dei sees women as mothers or housewives, and at university boys were given preferential treatment – in my year, a group of male students went on a trip to meet newspaper editors in the US; women were barred, on the pretext that it was organised by one of the Opus Dei male-only clubs.
The organisation has become more open following the publicity over Dan Brown's The Da Vinci Code. It has a website and a communications team aimed at making it look transparent.
But still, Opus Dei members don't proclaim their allegiance and often meet in anonymous flats or churches.
The group claims that it believes in freedom.
But obsessively insisting to teenagers that pre-marital sex is a sin, or suggesting they wear a celice to help them remain celibate, is far from freedom-enhancing.SIC: GCUK