Three centuries of slaughter of Donatists, Montanists, Arians and others, along with east-west controversies where those groups mixed, mainly in north Africa caused Christians there to welcome the Muslim invaders who were neutral on these schisms but stopped the “Christian” on “Christian” bloodshed. And it was mainly the orthodox of the east and west doing the killing with support of the Roman/Byzantine state.
No true Christianity is spread by the sword.
Philip Jenkins, called the “Jesus Wars”.
It was a time when doctrine was being put together, and heresy was being fought against. The period of the great councils.
While the violence was very wrong and horrible, got to remember that heresy has no place in the Christian faith ether. That has to be understood.