Here's one of the key points:
To commit heresy, one must refuse to be corrected. A person who is ready to be corrected or who is unaware that what he has been saying is against Church teaching is not a heretic.Depending on which defender of Rome you're talking to, you may be OK believing your Protestant heresy. I've come across some Romanists that say you are only committing heresy if you know that Rome is the true church, but still choose to believe something contrary to what she says. So according to these folks I'm not committing heresy because I don't believe Rome is the true church. Then there are those zealous defenders of Rome who long for the old days and realize the absurd qualifier just described is just that... absurd.
It's interesting to watch one of the less-ecumenical defenders of Rome on the Catholic Answers Apologetics forum clean up the ecumenical mess of recent Roman history:
That's for people who are absolutely clueless. They have to be innocently ignorant of this topic to qualify. It doesn't cover those who are NOT innocently ignorant. i.e. put little effort to learn the truth, or refuses to learn, or pretend to be ignorant, or are just hard of heart, or just plain stubborn, ( 1791 , 1859 ) they are NOT considered ignorant but culpable for their state. That's why the CCC states that Once someone "knows" 846 then they are required to act. And this knowlege is easy to find today. It's never been easier. (source)
For clarification Protestants are NOT the Catholic Church, no matter their stripe. Protestantism regardless the stripe, is listed in The Great Heresies "for those who knowingly and deliberately (that is, not out of innocent ignorance) commit the sins of heresy (rejecting divinely revealed doctrine) or schism (separating from the Catholic Church and/or joining a schismatic church), no salvation would be possible until they repented and returned to live in Catholic unity". We talked about the following just the other day. For those born outside the Catholic Church, aren't guilty of schism. But when they come to the knowledge of the Catholic Church, her founder, and necessity for being in the Catholic Church for salvation, would refuse to enter the Catholic Church, THEN they become guilty of that sin. Their Ignorance is no longer innocent. (source)