There's a book series called The Hammer and the Cross about an English bastard child of a noblewoman that resulted from her being taken by a Viking raid and later escaping back to her home. Then the Vikings invade to avenge the death of Ragnar (his 4 sons are each powerful Viking Jarls).

The way it handled the two religions clashing, where each was powerful based on how many followers they had, along with it being the first time I'd seen where Christianity isn't presented as the Underlying Truth but was just another thing. I realized that it was a metaphor for how religion actually worked. If enough people believe in something, it gains power. Christianity won through politics and warfare, not through truth. There wasn't anything special separating Christianity from other former religions we largely now refer to as myth other than the one Empire that united most of Europe declared it to be the truth and people were slaughtered until they went along with it.

That's when I stopped being a Catholic that just hated going to church and was an atheist at first, then later settled into agnosticism since who knows what's going on beyond what we can directly detect with our senses and tools.