Fereshteh Dibaj and her husband Reza Montazemi, a Christian couple in Iran, were released from an Iranian jail last October after spending 10 days in detention.

The charges against them are still unclear, but it is likely to do with their religious affiliation. They belong to Iran's little-known Christian population, a minority that constitutes less than one percent of the country's 70 million inhabitants.

According to Amnesty, Reza's mother, who owns the house in which they live, was reportedly ordered to sign a document two days before the couple was arrested, promising that no more Christian meetings for worship, prayer or Bible study would be held there.

It should come as no surprise that Tehran's religious authorities are concerned about the Christian community.

According to Christian organizations, the country is witnessing a huge spate of conversions from Islam to Christianity. The trend began shortly after the Islamic revolution in 1979, increased during the Nineties and has continued since President Mahmoud Ahmadi Nejad came to power in the summer of 2005.

The number of Christians in Iran who hail from Muslim backgrounds is approximately 70,000, based on estimates of churches and individuals. There are no accurate figures of converts. Discretion is vital in order to avoid the wrath of the religious authorities.

Fereshteh Dibaj is the daughter of Reverend Mehdi Dibaj, a Christian convert who spent 10 years in prison on grounds of apostasy. He was murdered in 1994, shortly after his release from prison.

Fereshteh's husband Reza reportedly converted to Christianity in his youth.

Flouting government orders, the couple continues to lead an independent house church in Mashhad, in northeastern Iran.

Christians in Iran break down into several groups. The larger and older communities include the Armenians, who immigrated from Armenia and Turkey several centuries ago, and the Assyrians, who likely originated from Syria and Iraq.

Smaller and more recent congregations include quite large numbers of converts to Christianity.

Under the Islamic regime that came to power in 1979, there has never been total religious freedom for the Christian population in the country, says Wilfred Wong, a researcher with the Jubilee Campaign, a pressure group that lobbies to protect the persecuted church.

"The situation has fluctuated between bad and worse," he says. "At the moment it's worse."

All agree that Christians who bear the brunt of persecution in Iran are the evangelicals and converts from Muslim backgrounds.

"This is not to say that the traditional non-evangelical churches have complete freedom. That would be na