Today we celebrate St Athanasius, and can somebody beam him down? The place is crawling with Arians again.
The simple version is that Arius–a Bishop of Alexandria–taught that Jesus Christ was not fully divine. Nearly the whole church followed this new teaching. It made sense, after all for lots of reasons it was a bit of a stretch to say that God himself took human form. Athanasius stood up for the historic teaching and suffered great hardship and persecution until he and his side finally prevailed.
In the church today the same battle is going on. Read more.
UPDATE: For a quick summary of the Christological heresies of the 4th century go here. Go on…learn something!