***Christianity spread rapidly and confidently when it was backed by force****
Thew first nation to fall to Christian armies was ARMENIA! then Greece and Rome! The the Christian Armies trampled England and Ireland, spreading with the sword clear to China with Nestorians! Then Russia fell to the advancing hordes of Christian armies on the march!
OH WAIT! There were NO CHRISTIAN ARMIES until the advance of Islamic armies! Only unarmed preachers and priests, armed with nothing more than the Sword of the Lord, the Word of God.
You should have put a sarcasm tag after your post. That is unless you were serious. You don’t have to have a “Red Cross” emplazoned on a sheild to be a “Christian Army”. The Catholic Church supported Orthodox Franks over Arian Visigoths centuries before Islam was even born as a Faith. Voila! Arianism is eventually crushed by force. Charlemagne executed pagan cheiftains who failed to convert to Christianity....this had nothing to do with Islam. In 392 (shortly after Christianity became the state religion of the Byzantines) Theodosius outlawed paganism. Over the next century force was used to drive all forms of Paganism out of the Empire until for all practical purposes it could not show itself in public.
Anyone who thinks thinks that Orthodox Christianity has spread (and remained dominant) SOLELY on the basis of its conversionary power is ignorant, delusional, or both.