Well Europeans have always been of huge followers of christianity. When Europe was going through their rough times of the black death and smallpox, many did reach to there faith in search of an answer. When they discovered what muslims where doing, because Muslims are big on spreading your faith as much as they can, so when Europeans and Muslims crossed paths, the Europeans did not like that there was another big realign and society interfering with there. I believe it was more of a pride thing.
During the Reconquista all of the mini kingdoms of Northern Spain and Portugal joined together as Christians to push out all of the Muslims. They later made sure that everyone was Christian and now they are still Christian today.
The emerging Russian kingdom adopted and modified the religion of Orthodox Christianity, which because the major religion of the country until the Communist Revolution in 1917.
