If you think about it, when Rome was Christian, Christianity was where Arianism was, and Arianism took Christianity's place. Christianity was once being followed only by a small group of people, and Arianism was ruling Rome.
Everything changed when Constantine converted himself and became Christian. Now everything was reversed, Christian ruled Rome and most of the known world, and Arianism was being followed by a small group of people. For me I found it as karma, for i think emperor Nero when he blamed the Christians for something he did.
