You can tell what happened by the ways in which Christianity tried to squash it. If you can't see the wind, you have to look which way the leaves rustle to find the source.
We know that the crusades meant people did not give up their religion or traditions without violence. We know that when Christians started printing things, they printed manuals on how to hunt witches, pagans, demons. We know that references to old traditions continue in art. I feel like I'm giving sources.