Founded in Spain in 1928, the movement has more than 80,000 members worldwide, many of them lay people but also hundreds of priests, bishops and even two cardinals who will be casting votes in Rome.
Its mission - to give lay people a dynamic role in spreading the word of God - enjoyed firm support from John Paul II, who championed the movement as a means of confronting the secularisation of society and reinforcing his conservative doctrine.
But Opus Dei - Latin for God’s work - has also been accused of secretive, cult-like practices, brainwashing of members into blind devotion and murky financial dealings.
“There is obviously some concern over whether the next pontiff will be open to something like Opus Dei,” said Anthony Figueiredo, a priest and professor of theology at Seton Hall University near New York, who was once based at the Holy See.
“I can be sure in this pre-conclave period, this is one of the areas they are discussing,” he said, referring to the secretive gathering of cardinals that begins next week.
Opus Dei is what is known as a “personal prelature”, which in practical terms means its leader, Monsignor Javier Echevarria, does not answer to any diocese, only to the pope himself.
It plays a central role in Dan Brown’s runaway bestseller, which has provoked unprecedented protest among Roman Catholic and Protestant conservatives.
The Da Vinci Code depicts the movement as a mysterious centre of political and economic power that tries to hide the historical truth on Jesus and Mary Magdalene - namely, that they married and sired a bloodline. An Opus Dei devotee commits the murder that sets the plot in motion.
Fr Figuereido said Opus Dei has “enormous influence” in the Vatican through sympathetic clergy members who staff Vatican offices. Vatican spokesman Joaquin Navarro-Valls is a member. But he has no guarantee of being reappointed. Nor do cardinals who held jobs as heads of Vatican offices.
Juan Maria Laboa, a priest and Catholic church historian in Madrid, said Opus Dei is so well established that the new pope might not be able to curb it even if he wanted to.
The two cardinals known to belong to Opus Dei are the archbishop of Lima, Peru, Juan Luis Cipriani, and Julian Herranz, a Spaniard based at the Vatican.
Another cardinal, the archbishop of Milan, Dionigi Tettamanzi, is known to be sympathetic to Opus Dei and is a possible papal candidate, according to Fr Figuereido and Spanish theologian Enrique Miret Magdalena.
A few years ago John Paul II transferred Cardinal Tettamanzi from Genoa to Milan, one of the world’s largest Catholic dioceses. And such a shift is rare for a church leader who was already a cardinal, Fr Figueiredo said.
“That is very significant,” he said. “He was almost saying, ‘this is my man’.”