Ever heard of the concept of inculturation, where the message of Christ becomes part of the local culture of a people?
This has been so ever since the days of the Apostles, with Pentecost and later the Council in Jerusalem which declared that Gentile Christians need not adhere to Hebrew customs like circumsicision.
Pope Gregory the Great declared that any cultural custom that didn't contradict Christian doctrine was to be included.
Now was this all just a political ploy, as many here are trying to argue? Well you could argue that; but as Richard Fletcher notes in The Barbarian Conversion: From Paganism to Christianity, to do so would mean projecting modern cynicism unto the past - when religious claims were very serious business.
This is especially true since a major justification for why many pagan customs were maintained after Christianization was because they bore a strong resemblence to the folk customs of the Israelites in the Old Testament.
It also ignores the important theology behind such practices. Pope John Paul II summarised it as such in his Slavorum Apostoli, commenting on the evangelizing efforts of Ss. Cyril and Methodius, and the model they set for evangelizing cultures in general:
"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."
This is in accordance with the claim set forth by St. Thomas Aquinas in the 13th century about how grace does not negate nature, but rather transforms it. Same logic applies to cultures.
As GK Chesterton noted, one does not cease to be human when one becomes Christian. Humans love festivels, pagan or Christian. And of course the chief boast of Christian Europe is that it is built on top of pagan Europe - particularly Greece and Rome. You might as well say our legs are of pagan origins too.
That's all I have to say.