Why was early Christianity incorporating into itself pagan mythic traditions?

Answer 1

Christianity started off as an offspring of Judaism, but as to gain popularity in the Roman Empire it started to merge with the pagan ideas of that time. It was not a single event that transformed Christianity, but rather a slow transformation to appeal the general population of the Roman Empire.

Answer 2

Many of the pagan traditions that were supposedly incorporated into the Church back in the 4th century, like Christmas from Saturnalia, did not come about through "Christianizing" the pagan festivals in order to attract pagans into Christianity. But they rather started out as alternatives to the pagan festivals, so instead of staying at home doing nothing whilst the rest of their town partied, the Church put on events to encourage people to focus on Christ, rather than being temped into getting involved in the pagan practices. Naturally as Christianity grew so did these alternative celebrations, even becoming bigger that the pagan celebrations.

We see the same type of thing happening today with Halloween, churches put on alternative parties to Halloween with the intention of taking the focus off the demonic and putting it on Christ.

As for the many pagan philosophies that are infiltrating the Church today, and have been since even before the 4th Century these are heresies.

Answer 3

To convert pagans by presenting Christianity in their terms.