Environmentalist and evangelical Christian Marina Silva has been confirmed as the Brazilian Socialist Party's presidential candidate, following the death of their previous candidate in a plane crash last week.

Former Socialist Party candidate Eduardo Campos died when his campaign jet crashed in bad weather in Santos, near San Paulo, on August 13.

Campos was mourned by tens of thousands at his funeral in the north-eastern city of Recife on Sunday. The cause of the accident is still unknown, although there have been no suggestions that it was a malicious incident.

A committed Pentecostal Christian, Silva said it was an act of "divine providence" that spared her from joining her colleague on the plane when she spoke to journalists in Recife on Saturday.

The 56-year-old former environment minister won nearly 20 per cent of the vote when she ran for president four years ago as a member of the Green Party.

Silva later left the Greens, and launched her own party in 2013 – the Sustainability Network – which failed to gain enough support to qualify for the election. She then decided to join the Socialist Party and became the nominee for vice-president under Campos.

Campos was third in the polls before his death, with about 8 per cent support, but Silva's nomination is likely to put the Socialists into second place.

The current president, Dilma Rousseff is still the favourite, although some polls put Silva slightly ahead if she makes it to the second round of voting. The first-round vote is scheduled for October 5, and the second on October 26.

While Silva may be behind in the polls, her journey into politics is a remarkable story of unlikely odds.

She grew up in the Amazon rainforest, one of 11 children born to rubber tappers. She only learnt to read at the age of 16.

As a teenager, she suffered from hepatitis and malaria, but was taken in by Catholic nuns who nursed her back to health. At the convent she was taught to read and was also introduced to liberation theology – a reading of scripture which prioritises the needs of the poor.

After working as a domestic maid, Silva became a teacher and environmentalist. Then at 36, she became Brazil's youngest senator ever, as well as being the first rubber tapper elected to the Senate.

She served as environment minister between 2003 and 2008 as a member of the Workers' Party, before resigning over difficulties implementing the government's environment agenda – a decision that was guided by principle.

"My aim wasn't to keep my job [...] It was an ethical choice. Political decisions always have to be guided by ethical decisions," she said in an interview with The Economist last year.

President Rousseff struggled to gain the support of faith voters at the last election, despite her Catholic background.

Silva appeals to both the progressive left and conservative right on some issues, though she is by no means guaranteed support from Christians.

Evangelical Christians make up between 20-25 per cent of Brazil's largely Catholic population.