The persecution of pagans and heretics by Christians beginning with Constantine was the greatest error ever to enter Christianity and it continues today. There was never any authority for it. God’s kingdom is not of this earth.
Not really.
Rather it helped to get the Christian faith spread even faster.
What you call “persecution” was the needed correcting of heresy and the promotion of “orthodox” or right Christian faith. There were a LOT of heresy at the time.
God’s kingdom is in BOTH in Heaven and because of the Christian faith, in THIS world as well.