Mithraism is frequently cited as a precursor to Christianity, a title which most Mystery Religions could also claim. The big difference between the mystery religions and Christianity was that some mystery relgions excluded women entirely, such as Mithraism, causing well-heeled Roman matrons with a pious frame of mind to explore first Judaism, and then Christianity. Also, unlike Christianity, they made no special overtures towards the uneducated, downtrodden and marginal elements of society. They were religions chosen by the emperors, not slaves. Much of the symbolism and many of the stories in the Bible may be traced to earlier myths of the Persians, Egyptians, and other people from the near east.
While some will say that the nativity scene isn't a pagan symbol, Mithra's birth was witnessed by shepherd and Magi, who brought gifts to his sacred birth-cave of the Rock.
Anyway, here are a couple of videos that will clue you in to the real "reason for the season".