As Christianity spread through Europe they found it was a lot easier to enforce conversions if they let people keep some of their existing religious traditions:
“The god of Abraham is your god now, his son Christ is too, who is also god. He’s your savior. You worship him now not these old pagan gods”
“But what about our winter solstice? The people love that celebration to mark the turning of the corner as the days finally start getting longer”
“Fine. You can keep that. You can even do all that shit with evergreen trees and the like. Just call it Jesus birthday”
“Thank you! What about the spring equinox fertility festival to celebrate the world coming back to life? That one’s a lot of fun. People will be very upset to see it go…”
“Fine. You can keep that one too. Focus is now on Jesus coming back to life. Just change the name a little so it’s not super obvious it’s just a modified version of Eostre. Also be more low key about the sex. If anyone bitches about it burn them at the stake”
“!!! That seems a bit unnecessary!!”
“Alright guy’s - we’ve got a heretic! get the bonfire going”
“Wait wait wait - I meant unnecessary as I’m sure the people will be thrilled to accept Jesus as their savior”