I know that ignorance plays a HUGE factor in this, but is it grave matter for non-Catholics to not be Catholic?
Is it intrinsically evil? (that is, if they have invincible ignorance will it be a venial sin or no sin at all?)
If someone is born into another Christian faith, I don’t believe they are committing sin by following that faith, any more than it would be sinful for any non-Christian to follow their religion. If they are unaware of their error, it’s not a sin. It’s very sad that they don’t realize how much of the true faith of Christ that they miss out on, but that’s a different subject.
However, Catholics who leave the Catholic Church to join another Christian church, is another story. They are culpable and guilty of at least a venial sin, depending on their reason for leaving. If they are poorly catechized and are truly unaware of the gravity of leaving the Church, then it
might only be a venial sin. But, if it’s because they willingly reject a particular Catholic doctrine that they do not wish to follow, either to get a divorce, have an abortion, or to feel free to commit any other sinful act without having to acknowledge that it is a serious personal sin (to ease their own conscience), then it would be a very grave matter and a mortal sin for them. They would be committing another mortal sin, on top of the mortal sin that they were trying to avoid taking responsibility for in the first place.
Are non-Christians sinning? To the extant that they have heard the Gospel and reject it, yes.
Non-Catholic Christians do not consider themselves to be rejecting the Gospel at all. Their interpretation of the Gospel may be faulty, but they are not really aware of their error, so there wouldn’t be any sin.
Schism from Christ’s Church is indeed a grave sin, but many do not recognize the Catholic Church as the Church created by Christ at Pentecost.
Schism cannot technically be committed by those who have never been Catholic. Schism is committed by a group of Catholics who break away from the Church, because they reject the authority of the Magisterium or the Pope, but still retain the basic tenets and doctrines of the faith (such as the Eastern Orthodox churches).
Non-Catholic Christians are the descendants of people who not only broke away, but also rejected at least some of those basic doctrines and tenets of the faith that they refused to accept for various reasons. The original reformers were culpable and guilty of heresy, because they committed that personal sin through an act of their own will. But, those who are brought up as followers in later generations, are not culpable or guilty of that sin.
At least, that’s the way I understand it. I could be very wrong, so I’m open to any correction of my statements that are not in line with Church teaching, but I believe they are compatible.