I think it just means heresy that is a mortal sin, or at least, that was how I intended it to be used.
For heresy to be mortal sin, it would have to be grave matter, which means it would have to be heresy about something significant, such as the nature of God and His salvific plan for humanity, and it would have to be taught, not just held in your own head.
It would also have to be done wilfully, not just through involuntary doubt or ignorance, and in full awareness that it was against the teaching of the Church. That’s where it becomes difficult, if you’re Catholic, the teaching of the Church means the teaching of Rome, but if you’re Orthodox, the teaching of the Church means the teaching of the Ecumenical Patriarchs. If you don’t know which Church is the Church, but you do know that Rome or Constantinople regards your teaching as heresy, are you in full awareness?
Has there ever been a case of disagreement with the two Communions that were that big though? Like, we mutually believe practically all of the major things. There are some nuts and bolts where we differ, but those seem like very small issues that lack essence.
On the issue of Papal Primacy, from what I understand, the Ecumenical Patriarchate already has acknowledged that the Pope has Primacy at least in terms of honour, but not yet in jurisdiction. This appears to be the only area where there’s a difference.
On the issue of salvation, the Catholic Church never actually taught that salvation was limited to Catholics only; though individuals claimed this, it was never an official teaching, and in fact Popes have taught the opposite. So, though I’d see why a lot of people would think this is a problem, in reality, it is not.
I know we differ on the issue of the Immaculate Conception, but as the Easterners love Mary as much as we do, I’d imagine that this wouldn’t be a stumbling block.
Now, on the issue of saints, a Byzantine Saint disputing a Latin Saint doesn’t seem to be such a big issue. With the Avignon schism, different saints chose different sides. There was also a case of a nun who was relatively recently-beatified who had disputes with her bishop, who himself was beatified too.
If an Orthodox Saint were against, say, the Immaculate Conception, perhaps this too might not be a problem. Even Thomas Aquinas didn’t believe in the Immaculate Conception. This of course isn’t to say that the Immaculate Conception is in any way wrong – it’s completely right – but it just wasn’t a required component of faith for Saint Thomas in his time just as it wouldn’t have been a necessary component for the Orthodox Saint in his or her time either. If a saint in his own time believed in things that would be heresy in a later time, the saint would not necessarily be at fault.
