The sign, a cross locked within a circle, was chosen by the Catholic group's founder, Saint Jose Maria Escriva, to embody his vision of Christianity at the heart of the world.
"This cross is a reminder the founder exhorted us to emulate the first Christians, moving in the world and converting others with our faith," said Guillermo, one of around a dozen men living in the comfortable but unobtrusive residence.
Founded in Madrid in 1928 when Escriva was a 26-year-old priest, Opus Dei - Latin for "God's Work" - is one of Catholicism's most dynamic and controversial groups.
It teaches ordinary Catholics to strive for saintliness through their work and has more than 84,000 members. Escriva was canonised in 2002 despite some opposition from liberal Catholics and the group continues to attract suspicion.
Critics accuse Opus of promoting right-wing beliefs and using aggressive recruiting methods. Some former members allege cult-like practices such as brainwashing and coercion.
Its notoriety soared with the 2003 bestseller The Da Vinci Code, where US author Dan Brown depicted it as a secretive society ruthlessly plotting to keep the Church conservative.
"So many lies are told about Opus, even by ex-members who lost their way," said Reyes Carreno, a mother of five who runs nursery schools. "We are just ordinary Christians."
Like three quarters of members, Carreno is a supernumerary, meaning she can marry and have children. She and her husband make donations to "the work" but she laughs at the idea it is a sect - saying members are free to do as they choose.
"There was a time when Opus was not so open...That may have given the impression of occultism, even though it never existed," said Carreno, 44. Opus became more accessible when Pope John Paul recognised its special status in 1982, she said.
The practice of corporal mortification, an attempt to imitate Christ's Passion, has aroused mistrust. Critics say Opus encourages flagellation and the use of the cilice - a belt tightened around the thigh with metal prongs pointing inwards which is used in some religious orders.
Members say the reality is much more normal: mortification can mean doing a job well when tired or doing a favour without seeking reward. Escriva said the best mortification was a smile.
"I do not flagellate myself or put on a cilice. It would shock my wife," said teacher Juan Manuel Saenz, 39, whose spouse is not an Opus member. "I have other types of mortification: praying while I feed my baby, for example."
In increasingly secular Spain, many people view Opus with mistrust. Some point to the wealth and influence of members and the group's own assets, including a $US42 million ($NZ63.05 million) New York headquarters.
Opus founded Spain's most prestigious business school, IESE, and executives at some of the country's top companies are said to be sympathisers. Spain's third largest bank Banco Popular donated 21 million euros ($NZ45.6 million) last year to charities linked to Opus.
Opus' Navarre University has been responsible for producing some of the country's highest achievers. Its respected journalism school claims to have produced more than half of the editors of Spain's national newspapers. The Vatican spokesman, Joaquin Navarro-Valls, is a prominent Opus member.
Opus has been accused of wielding political clout. During Francisco Franco's dictatorship, members formed the backbone of a "technocrat" cabinet from the late 1950s which dragged Spain from economic collapse - helping to prop up the faltering regime which subsequently survived to Franco's death in 1975.
More recently, a former defence minister during Spain's 1996-2004 conservative government said he was a member. Other figures in the Popular Party, which backed conservative social policy and a church role in education, were linked to the group.
"I defy anyone to prove Opus Dei has ever supported any government," said Opus spokesman Manuel Garrido. "Individual members are free to take part in politics, like any Catholic, but Opus does not take positions on social or political issues."
The smartly-dressed Garrido is a "numerary". He lives in an Opus house near Madrid's business district, is celibate and donates much of his income to the group, his "family".
"God asked me to renounce human love and dedicate myself to him," said Garrido. "It is comfortable to have lukewarm beliefs but Opus wants us to show we're Catholic, helping others."
Some ex-members have attacked the group's methods. Maria del Carmen Tapia in her book Beyond the Threshold depicts Escriva as authoritarian and accuses Opus of brainwashing young people.
While Opus Dei has often dismissed Tapia's criticisms as outdated and inaccurate, more recent former members still complain about the group's methods.
"When I wanted to leave they did not want me to go and tried to keep me. Eventually, they let me go," said a former member who joined Opus at the age of 19. He asked not to be identified.
Opus priest and theologian Jose Carlos Martin, in charge of promoting other members of Opus as possible candidates for sainthood, said liberty is essential.
"Nobody in Opus Dei is obliged to stay," he said. "If someone leaves Opus there is no trauma - they continue their working life. The only trauma could be that they have not been faithful to a call from God."
The Opus Dei school Tajamar was founded in 1958 at Escriva's request to tackle a lack of education in one of Madrid's poorest suburbs. It is an answer to critics who complain that Opus Dei ignores the poor in favour of recruiting among the elites.
"There is a lot of demagoguery with this issue. Opus does a lot of social work at its schools," said Garrido. "Opus is not an NGO, although it has many members who work in NGOs."
Starting from a cow shed amid tumble-down shacks, Tajamar has 1500 male students from three to 18. Its manicured campus houses one of Madrid's best graphic design schools.
"Of course we try to instil the values of Opus Dei," said school director Alfonso Aguilo over lunch. "But we also have students who are Jehovah's Witnesses and Muslims. They can choose not to attend religion classes."
Mohammed Karimian, an Iranian whose father works in his country's embassy in Madrid, praised the teaching at Tajamar.
"I have never been made to feel different because I am Muslim."
Escriva began Opus by recruiting university students and the group focuses on the educated when it starts in new countries.
"Where do you start in order to reach all social classes?" said Martin. "If an intellectual accepts God, that person is going to have a rapid influence in society."