Heresy is like fire, it burns all around. It was the Orthodox Christian persecution of “heresy” that made Islam a worldwide religion. But for the persecution of the Ghassanids Mohammed’s religion would be a quaint tribal anachronism constrained to the Saudi Arabian peninsula.
Of course, it was the persecution of heresy that gave us America and religious tolerance; later clearly enumerated in the First Amendment.
Tragically, it was the persecution of Catholic’s as heretics that gave us our government school system. Something we’re suffering for today. So it’s a mixed bag.
Absolutely. The extraordinarily rapid Muslim conquest of Syria, Egypt and North Africa correlates exactly with the areas where Christian "heretics" of various types were being severely persecuted by the "Orthodox" Christians in control of the Byzantine Empire. These sectarians, logically and accurately, decided they would have greater religious and personal freedom under the Muslims than the Byzantines. They at minimum stood aside, and apparently often covertly or overtly supported the Muslim conquest. Same for the Jews in these areas.
In the long run, it turned out they did not choose wisely, but there was no particular way for them to know this at the time.
Even the complete collapse of the Visigothic Spanish kingdom as the consequence of a single battle may be related to religious dissension and persecution. It took the Romans two centuries to conquer the same territory, and the Christians seven centuries to reconquer it.
I am always particularly amused by Protestants who "know" what constitutes heresy. Catholics, with their single source for approved doctrine, at least have a logical basis for pronouncing those who differ as heretical. (Some) Protestants, with no such authority, confidently denounce Catholics, Orthodox and other varieties of Protestants as heretical. Which means they have set themselves, as individuals, up as infallible interpreters of God's Book and Will.
Your timeframe for this development, please?