What are the reason Christianity spread so fast in Europe during the middle ages?

Today a faithful reader sent in the following question for us to answer:
Is it because ancient European’s cult practices are like being an atheist and that is why Christianity influences them so fast?
I was also thinking of today’s situation where Europe is becoming more secular and Islam is spreading in Europe. For this reason, Muslims can easily influence and convert Europeans into Muslims as well, isnt it? Please give me some details.

