Concerning the adoption of pagan customs into Catholicism, it's called "inculturation". Contrary to popular polemics, this does not negate Christian truth when one actually bothers to understand the mentality behind this, as summarised by John Paul II - when commenting on the evangelizing work of Ss. Cyril and Methodius:
We Catholics don't tell people to give up their native cultures and customs, but rather to transform them with the truth of the Gospel."The Gospel does not lead to the impoverishment or extinction of those things which every individual, people and nation and every culture throughout history recognizes and brings into being as goodness, truth and beauty. On the contrary, it strives to assimilate and to develop all these values: to live them with magnanimity and joy and to perfect them by the mysterious and ennobling light of Revelation."
-- Slavorum Apostoli
Not only that, Pope Gregory the Great even decreed that any custom that didn't contradict Christian teachings could and should be preserved. I mean seriously, what's so un-Christian about decorating a tree?