The greatest spread of Christianity was in its first 300 years, when Christians were all martyrs and there was NO FORCE on our side, .... except maybe for the Holy Spirit.
Actually, it wasn’t. Christianity spread slowly compared to the period of it’s adoption by Constantine. Also, consider how rapidly it spread after Clovis converted. Charlemane imposed it by force on neighboring pagans. Consider the conquest of Eastern Europe by the Teutonic Knights and the reconquista of Spain. Finally, the spread of Christianity to the Americas was accomplished as much by gunpowder as by evangelization.