Catholics, Orthodox, and Protestants who affirm, explicitly or implicitly, the doctrines of the Nicene Creed (we can debate filioque another time :> ) may differ in numerous ways--many of which might be considered adiaphora--but when it comes to the heresies as listed in this article, they are just as anathema to the "orthodox" outside of the Catholic Church as to those inside.
Do the Eastern Orthodox maintain belief in the notion of original sin?
I am pinging TheReader to this, as he is so learned.
I have a notion that original sin comes from the teachings of St Augustine, and is not in the Eastern Church. They believe baptism is entry into the community of faith, rather than a washing away of original sin. At least, that is what I have heard.
I think the notion of killing heretics, as in the crusade against the Cathars was a heresy imported from Islam.
A military leader asked how he could determine if a Cathar was a heretic.
“Neco es omnes, Dio se agnoscet”. was reportedly the response that the learned monk gave.
Kill them all, G-d knows his own.