If they could have bet on which footballer was most likely to walk into the headquarters of the International Evangelical Community, a pentecostal temple located in Rio de Janeiro's Flamengo neighbourhood, Frederico Guedes would probably not have been high up on the congregation's list.

But the striker, also known as Fred, has recently become their newest member; the Fred who is almost as famous for his heavy partying and womanising as for his goals for Fluminense and the Brazil national team is now a devout Christian who on 12 March was pictured walking into the temple with a Bible in his left hand.

Even his club manager found it hard to believe. Renato Gaúcho, whose hell-raising antics might have slowed down since his retirement in the 90s but still make him a famous lothario, did not hold back when commenting on Fred's conversion. "Is he going to church now? Well that would make me a priest then because we are both up to the same shenanigans. That kills me, to hear he's an 'Athlete in Christ' now," said Gaucho, roaring with laughter.

That last phrase gave away more than banter, though. Although religion pretty much drenches every aspect of Brazilian life, its role in sports and politics is mired in controversy. Still the country with the biggest Catholic population in the world (123m out of 190m people), Brazil in the past few decades experienced significant gains from evangelical denominations.

In the past 40 years the number of Brazilians declaring themselves Catholics decreased from 92% of the population to 57%. Evidence of this swing can be seen in the Brazilian parliament, where 15% of MPs openly defend an evangelical agenda, and football has also become a highly visible arena for militancy.

In 1984, a group of footballers led by Baltazar, a cult-hero centre-forward nicknamed "God's Striker" thanks to his professed faith, founded the Athletes in Christ, an association of protestant sportsmen that would prove to be one of the most influential power groups in Brazilian football over the next decade.

Their grip over the Seleção was specifically felt at the 1990 World Cup, where they would meet regularly for prayer, invariably led by right-back Jorginho, who years later would be the assistant manager in the 2010 World Cup campaign.

Italia 90 ended miserably for the Seleção, amid tales of infighting and a mutiny over prize money and sponsorship deals, and did not work as good publicity for the group.

The Seleção has hardly been a secular environment: in 1958, a pep talk about how the blue away kit had the same colour of Mother Mary's shroud worked wonders to calm down Pelé and co's nerves when the team learned before the final game that the hosts, Sweden, would be playing in yellow shirts. Celebrations on the pitch after Roberto Baggio sent a penalty into the Californian sky in 1994 included prayers on the pitch, a ritual replicated in 2002 – in fact, the manager Luiz Felipe Scolari carried around a Mother Mary statue that "blessed" the dressing room and actually followed him to the Portugal job.

However, militancy never went down well and the issue became thornier when Kaká, a devout Christian who didn't shy away from talking about God and faith, became Brazil's biggest player of the second part of the Noughties. Alongside the defender Lúcio, their effusive use of apparel with religious messages (does anyone remember the 'I belong to Jesus' boots) ruffled feathers at Fifa, which officially told off the Brazilian Football Confederation after the 2009 Confederations Cup final in South Africa.

A year later, the Seleção not only had Jorginho as assistant manager and vocal evangelist who would threaten critics with divine retribution; a preacher was an informal addition to the technical staff. Four years on, the picture could not be more different.

These days, few people outside Brazil know that Neymar is a protestant, alongside the captain Thiago Silva and Chelsea's David Luiz, even though it is quite normal to hear them thanking God for achievements and assorted blessings. There are no organised activities while on national duty and even talking about faith became a bit of a hot potato. "Let's say we keep the praying to ourselves, without trying to make a big issue out of it", says one Seleção player, who asked to remain anonymous.

After months of struggling with his fitness, Fred is allowed to resort to any comforts he wants. But his decision to "do God" publicly – the striker has been photographed in the Fluminense training ground with bilingual religious shirts – is certainly something that will send chills down the spines of some Brazilians before the nation stages the World Cup this year.