Iran recently changed the law to criminalise "deviant education or propaganda", and it is already being used to persecute Christians.
Article 500-bis was signed by outgoing president Hassan Rouhani in February and penalizes "sectarian activities", and "any deviant education or propaganda that contradicts or interferes with the sacred Islamic shari'a, will be severely punished."
Christian Solidarity Worldwide (CSW) reports that the new law is being used by the authorities to target "wrong cults" - groups that deviate from Twelver Shiism, the faith basis of Iran's Islamic Republic.
Violations carry prison sentences of between two and five years, a voting ban of up to 15 years, and heavy monetary fines.
In the past, Iran has used national security laws to arrest and imprison Christians, but this week, three Christians were made to stand trial under Article 500-bis.
Amin Khaki, Milad Goudarzi and Alireza Nourmohammadi, from Karaj, northern Iran, are all from Muslim backgrounds.
The Christians were charged with "sectarian activities" and "engaging in propaganda against the Islamic regime".
They were made to stand trial without legal representation after judge Mehdi Zeinali claimed that their lawyer had not registered with the court as their legal counsel.
CSW said the lawyer had in fact fulfilled all necessary requirements 10 days before the trial began.
The human rights organisation said this week's trial showed that the Iranian authorities are using the new law to go after evangelical and reform movements, as well as conversion to Christianity.
CSW's Founder President Mervyn Thomas said: "The new amendment severely impairs Iran's ability to fulfil its obligations under the International Covenant on Civil and Political Rights to promote, protect and uphold the freedoms of religion or belief, and of expression.
"These rights are already under pressure, and this amendment is likely to make the situation even more difficult for religious minorities in the country.
"We call upon the Iranian authorities to repeal this and similar laws which hinder the full realisation of fundamental human rights for the Iranian people, and to end the relentless campaign of harassment of Christians and other religious minorities through the judicial system."