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.
http://www.impantokratoros.gr/spread-christianity-persecutions.en.aspx
If that doesn’t convince you, then I concede the argument.
We will find the truth when in front of Christ.