I've quoted him before.
"Even if one should give away all his possessions in the world, and yet be in communion with heresy, he cannot be a friend of God, but is rather an enemy." +Theodore the Studite
I went and read a little on him-- sounds like the big issue in his day was between people who thought icons were ok and people who thought they violated the commandment on graven images. Hmmmm. Yup, that's worth breaking communion over. NOT. (Although I know it's been done, over and over again.)