I have frequently wondered why the varied peoples that make up what we now call Europe all abandoned their tribal Gods and took up what is ultimately a Jewish offshoot, from the Middle East. I know that ever since Rome Christianized it was being pushed outwards by great powers, but was that really all there was to it?
Can you narrow down your interest a little bit? The spread of Christianity across Europe was a process that took over 1000 years and differed significantly depending on when and where the process was happening.