It wasn’t by the sword, though that did come first. It wasn’t by persecution. It was through the pressures of peace and finances.
Back in the 7th and 8th centuries, Muslim conquerors didn’t aim to destroy Christianity. They simply wanted to control it. However, they did manage to extinguish the church’s witness. And it happened by offering security and financial stability.
Before the rise of Islam, Christians in ancient Persia experienced perhaps the most intense physical persecution any group of Christians have ever experienced. The church in Persia grew steadily, as missionaries from Antioch and Edessa ranged further east. Naturally, this caused some tension with the leaders of the established state religion, Zoroastrianism. The conflict between it and Christianity was often more acutely felt by the Zoroastrian clergy than that experienced by pagan priests in Rome. The clergy (called magi) often led in the persecution of Christians.
A consistent, region-by-region persecution of Christianity started in Persia around 340 AD and lasted for over 40 years. It’s estimated that as many as 190,000 Persian Christians were martyred during this time – far worse than anything ever experienced in Rome (Samuel Hugh Moffett, A History of Christianity in Asia, Vol 1, 144). And yet, in all this time, though the church was decimated in terms numbers, conversions continued and the Gospel spread. Like a twisted version of Whack-a-Mole, every time the authorities cut down one Christian leader, another would spring up somewhere else.
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