Everyone thinks Christianity is pagan in origin. Simple truth is it isn't.
Sorry but you weren't given chocolates and a bunch of flowers for being a Christian in the 300 years after Christ's death. You were crucified, thrown to the lions etc.
When Rome decided to embrace Christianity, why place these celebrations on pagan holidays when most people were already pagans.
It's called externalisation and most people are not aware of the origins of Paganism.
Just when you thought you knew something.