Matthew's Journal

It's to do with from where the Holy Spirit proceeds in the Nicean creed, which is the Christian, um, manifesto, declaration, thingy written in the second century AD. The bible says that the Spirit proceeds from the father, but doesn't explicitly say that it proceeds from the son, though there are clear implications of such. So Augustine in the third century (I think) added "and the son" to the from where the spirit proceeds (though not subordiante from either, and sharing intent, etc). The eastern orthodox church didn't like that so they didn't add it. For some reason, during the great schism in the eleventh century, when the west and east fell out, this became of great importance (as far as I can tell they just wanted to fall out, and because this was a difference it became of great importance). Some people even say it was originally added (or not added) in order for churches not to have complete agreement in creed, to serve some kind of nefarious church-political end.

Though it's been a long time (well over a decade!) since I studied such stuff.