I just wonder what political power a fallen empire can offer though? The general pattern is that empires and civilisations fall but religions survive.
Christianity didn't need Rome, in fact it hasn't had Rome for a long time. There are claims that Christianity weakened the empire.
The political power that organised Christianity had acquired was sufficient for it to persist after the fall of the Western Roman Empire: Christianity was active elsewhere you know at that time, such as the the Byzantine/Eastern Roman Empire, and as it survived in morphed into various subsets along the way while still exerting political power and social influence - but also friction and conflict.
The so-called 'wars of religion' that began in the early 16th century and continued until well into the 18th century, over a millennia after the fall of the Western Roman Empire, and well before that Christianity was a active political force (such as in Anglo Saxon times).
Thankfully, in the wee bit of the planet I inhabit, Christianity nowadays has little power or influence, and is a minority activity.