Iran's government allows some registered churches to function with limited freedom. These are the historic Armenian and Assyrian Orthodox churches. They have served their own ethnic groups and been a faithful witness in Iran for centuries. 

These Christian communities are tolerated partly because they are part of the ancient fabric of society, but mainly because they worship in their own languages, rather than the national language of Persian (Farsi). Therefore, they are unlikely to convert Iranian Persian-speaking Muslims.  

Before 1979, a couple of dozen Protestant churches had also registered with the government. Their meetings were held in Persian and so Iranian Muslims could attend and understand the services. After the revolution, these churches came under increasing pressure to cease their activities.  

Today almost all the Protestant churches have been forced to close their doors. The few that still are allowed to operate are small and closely monitored. No new Muslim-background members are allowed to join.