Read "On the Incarnation" by Athanasius of Alexandria. The whole work (which is only just short of 90 pages long) can be found on Google books. It's a great summary of the Christian Gospel.
Because books have a tendency to slip ideas in the back door when the front one is locked. Why do you think Wiccanism, Paganism, and Witchcraft are on the rise?
That's actually why the Eastern Orthodox use leavened bread for their Eucharist. When they learned the Latins were using unleavened wafers shortly before the Great Schism, they saw it as Judaizing.