Have you ever lived in Iran? If not then the Iranians you know must be ex-pats and thus not representative of the situation in Iran.
Given that you made the claim I might ask you the same thing.
http://www.elam.com/articles/The-Church-In-Iran/
Remember that "exponential" doesn't mean a large percentage if it starts out small. But if as estimated in 1979 there were only 500 Christians in Iran that converted from Islam, and even if it's only 0.5-1% of the population today, in a country of 79 million, that's a huge growth.
And what happens when the majority of the host society, including those who don't like the particular regime in power, doesn't like missionaries? Even the Dalai Lama tells his people (without ever telling westerners) to support the Chinese government cracking down on covert missionaries. Is the will of the people more important or is a rather nebulous "right" more important?
Then they don't convert. I have no intention of ever becoming a Mormon or Jehova's Witness, but I don't need to ban them from doorknocking or visiting me to ensure this doesn't happen.
I’m just challenging the assertions that you make. Linking to a biased source whose facts can be easily checked doesn’t help.
For example, you are required in Iran to register as one of a number of religions; atheist and agnostic Iranians are not recognised by the government. So we are able to estimate the number of Christians, but not necessarily the numbers of those who hold no faith. Christian churches existed prior to the revolution particularly in Armenian minorities which were Christianised centuries ago. Their population is about a quarter of a million. The idea that there might only have been ‘500 Christians’ prior to 1979 is ludicrous. The size of the Protestant Christian community is estimated at just 10,000. Many may practice in secret of course, but the idea that they run into six or seven figures and undermine the regime is wishful thinking. There is no evidence that their growth is exponential.
You also ignore the history of Iran and the practices of Shia Islam in particular and make the common mistake of equating Islam as practiced by the people with the Islamic regime. Why would people who oppose the regime dispense with their faith given that until 1979 their faith was compatible with a modern, increasingly secular capitalist economy? Those who are anti-regime are also devout Muslims.