After the warlord ruler Constantine (later to be Saint Constantine) won a battle by supposedly hoisting the Christian cross, he convented to the creed of the gentle Jesus (but didn't "put up his sword") and got the enrire Byzantine empire to likewide convert.
And so the creed of persecuted person and gentile slaves became a state religion.
In doing so it lugged after it like a spoor many of the pagan rituals, beliefs and celebrations, It made the transition to Christianity a lot easier for the average pagan guy and gal.
Once, in the Armstrong publication, The Plain Truth, there was a good article as to how internal evidence of the Bible (shepherds tending flocks of sheep, the position of the Star, etc) deomonstrated tht JC was born,not in late December (by our calendar) but sometime in October.
The birthday of the Savior was moved up to co-incide with the Winter Solstice, which takes place (by our calendar) on around December 21.
This was a time the pagan peoples, in the winter, would hope for a renewal. And do all kinds of pagan rituals to keep themselves happy in the cold. They made offferings to the Sun God, to return to them someday.
The old pagan Germanic tribes would bring an evergreen tree into the house and decorate it with brightly colored cloth. Thus the pagan origin of our Christmas tree.
Ditto St Nick, or Father Christmas, or Kris Kringle: Santa Claus,in his bright red and white get-up. Paganism survives in our hearts.
No big transition. The pagans at this time praised and worshipped the Sun God. Christians do the same, for the Son of God.
Easter, according to the old English religious historian, The Venerable Bede, was originally a spring celebration of a Germanic goddess of fertility, named Eostre.
Same deal. Spring is the celbration of new life, of resurrection.
Flowers and plants in bloom, the earth coming to life after the long, cold winter.
Eostre, Easter. And those delightful pagan symbols of fertiity: eggs, rabbits. If you be a Christian, and send your child on an Easter egg hunt, dressed in rabbit ears, you are honoring your pagan ancestors.
If we in the West are all Chistians (in our way of thinking, regarless of our faith or lack of same) we are also all pagans.
The enviromental movement encompassses a lot of the old paganism, the nature worship. As in "tree hugger."