I have to ask what you think 'rapidly' means in this context. Because I am not sure that the premise is correct. The Christianisation of 'Europe' took place over a period spanning more than 1000 years long.

It wasn't rapid at all. During the middle ages, post AD1000, there were still missionaries working on the christianization process expecially in the north and in very remote areas. Don't confuse urban environment with rural one. As far as we can tell the rural population never really gave up their pagan roots completely.