Credo ... in Spiritum Sanctum, Dominum et vivificantem,
qui ex Patre Filioque procedit,
qui cum Patre et Filio simul adoratur et conglorificatur,
qui locutus est per prophetas.

I believe ... in the Holy Spirit, the Lord and Giver of Life,
Who proceeds from the Father and from the Son,
And with the Father and the Son is adored and glorified,
And was spoken through the prophets.

You may recognize this as the Nicene Creed. Back in 325 Emperor Constantine tried to get all the bishops in his empire together to declare what the doctrine of the faith would be, and the Nicene is the end result. One of these days I'll make a post explaining each line because they're each connected with a controversy in the early church, but today I want to focus on that filioque.

Nowadays in America we focus on Protestantism and Catholicism as the two big contenders of Christianity, but back in the day before the Reformation it was between Catholicism and Orthodox. The two were fighting for influence all over Eastern Europe, with sometimes the Orthodox winning (Russia), sometimes the Catholics winning (Poland). The Orthodox Church clearly outmatched the Catholics; whilst the Western Roman Empire was collapsing under the barbarian invasions, the Eastern Empire remained strong for centuries. However there was a sense of a single, universal church. Sometimes the Orthodox patriarch would fight with the pope, or sometimes there would be a theological argument in the east that went completely over the head of the people in the west, but they really tried to keep it together.

The two finally split in 1054, and one of the reasons was this little word filioque. If you look at the Greek version of the Nicene Creed, there is no "and from the Son;" it only says the Father. The problem is no one really knows how to deal with the Holy Spirit. Although it's mentioned several times throughout the Bible, it's not really defined what it is. When the idea of a Trinity emerged, the Holy Spirit was thrown in there but its exact relation with the Father and Son is really fuzzy. Filioque is an expression of this: In the East, the felt the "ownership" was the Father, but it came through or was sent by the Son, whereas in the West they said both the Father and Son were involved in it. I'm simplifying centuries of debate, but that's the gist of it.

Again, this isn't what caused the East-West Schism, but it was definitely a contributing factor because people got really fucking angry about it. I want you to imagine a room full of scholars screaming at each other about something, now imagine what they're upset about is what the nature of God is. Today this seems like such a small, minor issue, but that's probably because few are asking deep questions about their religion nowadays, and instead focusing on minor shit like whether there are saints or not. And probably for good reason because we've learned from the Fourth Crusade, from the Reformation, from the Thirty Years' War what internecine battles in a religion can fucking do. It's just better to let sleeping dogs lie.