Christianity has a long history in Iran, although it has always been a minority religion relative to the majority religions (Zoroastrianism before Islamic conquest and Islam after that). There are around 400,000 Christians and at least 600 churches in Iran. Many old churches remain in Iran from early days of Christianity. For example, St. Mary Church in northwestern Iran is believed that is the second-oldest church in Christendom (dates back to 642 A.D.), after the Church of Bethlehem. The famous Italian traveler Marco Polo described this church following his visit. You can read about churches in different Iran cities including Tehran and Hamedan, Isfahan and Shiraz, Mazandaran, East and West Azerbaijan, Qazvin, Kermanshah and Ahvaz in these links.
The number of Christians in Iran was significantly boosted through various policies of subsequent kingdoms. For example, in 1606, King Abbas I settled some 300,000 Armenians within Iran, as well as establishing their own quarter in Jolfa district of the then-capital Isfahan, which is still largely populated by Christian Armenians. The Armenians have their own distinctive culture and language.
In the early 20th century, once again Iran extend Christian population was boosted. This time due to the Armenian Genocide (1914-1923) and tens of thousands of refugees poured in Iran.