It seems to me there is some confusion here on the part of several posters as to what the original questions is really asking. "How much of Christianity is based on Paganism?" If we are talking about the core doctrine of non-denominational Christianity as recorded in the Gospels of the New Testament and the subsequent epistles of Paul and others to the early churches, the only non-Judaeic influences I can see are perhaps some remnants of Hellenistic humanism. It's an entirely different story if we are talking about rituals, liturgy and peripheral doctrines e.g. sainthood (hagiography), celebrations of feasts and even the centrality of the Holy Trinity to Christian thought. Here the basis is almost entirely and completely non-Christian and non-Judaeic. (I don't like to use the word 'pagan'; it has unwelcome connotations to some.)
The emphasis on the Trinity, for example, came about largely as an attempt to make Christianity acceptable to the Celts, as most of their gods and goddesses already had a tri-partite nature. Missionaries could say, "You're right. The god we teach you about is also triple-natured -- father, son, holy spirit." But that doesn't mean that the concept was derived
from any Druidic source. It just didn't have any large-scale importance in Christian thought until it became necessary to start stressing it.
Virtually all the Christian holy days that are celebrated today -- both feasts and fasts -- derive from sources much earlier than the time of Jesus of Nazareth. We may claim that the Easter holiday celebrates the Resurrection, but, in fact, all the appurtenances of the celebration go back to the Germanic spring festival of Oester, which was a fertility feast and is therefore symbolized by such fertility icons as eggs and bunny rabbits which are notorious for breeding like...well, you know. Christmas, likewise, is an ancient pre-Christian feast. In the Roman Empire it was the Saturnalia, the mid-winter celebration of the Solstice when the days start to get longer again. In the Scandinavian countries (this one's for you, Saab!
) it was Jul (pronounced Yule) and included all the ceremonies of gift-giving and huge logs on the fire and evergreen boughs to symbolize the rebirth of the longer days. This is true of literally every single holy day you can point to in the Christian calendar. Saints simply took the place of demi-gods.
But, all that said, it would foolish to claim that, therefore, Christianity is just an offshoot of Paganism. It is an offshoot of Judaeism, yes, but the European pagan traditions were added on later as the Chistians began to proselytize the new religion. As to the connection between Christianity and earlier Near Eastern beliefs, e.g. the one exploited in that original video, Judaeism grew out of the same traditions as all other Near East religions. Abraham left Ur of the Chaldees in Mesopotamia to found what eventually became the Jewish religion. He was not yet a Jew. He was a Hebrew and, reading the Torah or Old Testament, it isn't quite clear just what his beliefs and the beliefs of his people were. It's a sure bet they were not yet monotheistic and there's every evidence that they were accustomed to performing human sacrifice just like their neighbors. (Didn't you ever wonder about Abraham so blithely accepting the fact that he had to sacrifice his son Isaac to Jehovah? That story is, in fact, an allegorical eplanation of how the Hebrews ended this practice.) It's hardly surprising, then, that certain core myths would be present in the thinking and belief of all the Semitic peoples of the region.
Again, this is not the same as saying that either Judaeism or Christianity derive
from these sources. They are merely sources shared in common. Certainly, the notion of a virgin birth is not original with the followers of Jesus. It was a case where the claim had to be made if the missionaries were to be taken seriously as preaching the word of God incarnate. In the belief structure of the first people they preached to, those of the Levant and Asia Minor, nobody could claim to be a god if he was not
born of a virgin.