Fastest growing religion in Iran is Christianity

 Newsweek:

Something religiously astonishing is taking place in Iran, where an Islamist government has ruled since 1979: Christianity is flourishing. The implications are potentially profound.

Consider some testimonials: David Yeghnazar of Elam Ministries stated in 2018 that "Iranians have become the most open people to the gospel." The Christian Broadcast Network found, also in 2018, that "Christianity is growing faster in the Islamic Republic of Iran than in any other country." Shay Khatiri of Johns Hopkins University wrote last year about Iran that "Islam is the fastest shrinking religion there, while Christianity is growing the fastest."

This trend results from the extreme form of Shi'ite Islam imposed by the theocratic regime. An Iranian church leader explained in 2019: "What if I told you the mosques are empty inside Iran? What if I told you no one follows Islam inside of Iran? ...What if I told you the best evangelist for Jesus was the Ayatollah Khomeini[, the founder of the Islamic Republic]?" An evangelical pastor, formerly an Iranian Muslim, concurred as far back as 2008: "We find ourselves facing what is more than a conversion to the Christian faith. It's a mass exodus from Islam."

As a clandestine phenomenon, the practice of what are sometimes called Muslim Background Believers (MBBs) lacks clergy and church buildings, but instead consists of self-starting disciples and tiny house churches of four to five members each, with either hushed singing or none at all. Its lay leadership, in striking contrast to the mullahs who rule Iran, consists mainly of women.

In another contrast to the government, Iranian MBBs tend to be fervently pro-Israel. They are, explains a documentary, "bowing their knees to the Jewish Messiah—with kindled affection toward the Jewish people." Converts have even expressed a hope to build a "resistance church" in Iran to counter regime threats to Israel.

Given the Iranian house church movement's underground nature, estimates of its size are necessarily vague. Open Doors found 370,000 MBBs in 2013 and 720,000 in 2020; Duane Alexander Miller approximates as many as 500,000, Hormoz Shariat at least one million and GAMAAN even more than that.

The mullahs have usually responded with predictable repression that includes prohibiting Christian missionaries and Gospel preaching. The U.S. State Department reported in 2012 that "government officials frequently confiscate Christian Bibles and pressure publishing houses printing Bibles...to cease operations." Also, Christians "reported the presence of security cameras outside their churches."
...

The Christians do face persecution for their beliefs in Iran.   Hundreds are arrested and face extended interrogations.  The Mullahs do see the Christians as a threat to their theocracy.  BTW, the Christians do favor Israel.