The common opinion held by theologians is faith in the Trinity and Incarnation are necessary,
It's a common THEORY but it's not unanimously agreed upon.
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therefore it flows logically that God enlightens these people who appear to die invincibly ignorant of the Faith.
As the theory, goes, you are correct. The key word is they "appear" to die in ignorance. Since God enlightens them, then they do not die in ignorance, but in 1) knowledge of the Faith, and 2) acceptance of the Faith.
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What I am debating with you (and with many others) is your imprecise and quasi-heretical language. It is wrong to say that an invincibly ignorant person, who is sincere and loves God, can be saved. Absolutely, positively they cannot. It is only accurate to say that those who know the faith (incarnation/trinity at a minimum), desire it, desire baptism, can be saved (assuming, which is debatable, that God would not provide them baptism, as He has done miraculously in many other cases. But I digress...).
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The point is that ignorant people cannot be saved, but only those who are FORMERLY ignorant, who have been ENLIGHTENED by grace, are saved. Thus, those who are formerly ignorant, are now Catholics before they die. Thus, as is catholic doctrine, only good Catholics are saved.
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In the very next sentence he affirms that there are those struggling with Invincible Ignorance of the True Religion, he then says by divine light and grace such a person if meeting the requirements could achieve salvation.
Yes, they can achieve salvation, IF THEIR IGNORANCE IS ENLIGHTENED and thus NO LONGER EXISTS. But they cannot be saved IN SPITE OF, CONTRARY TO, or WITH their ignorance intact.
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Pius IX's language here (at least in the English translation) is sloppy. The way it's translated, makes one think that an ignorant person can be saved, when that's not what he's saying. Either way, this encyclical is not infallible, nor is it teaching doctrine formally, so it's Pius' theological opinion, not formal Church teaching. He's correct in a sense, but he was also a big liberal for much of his life, and his imprecision and pre-Vatican 2 "salvation for all" type of thinking is apparent here and in seed form. I critique his orthodoxy here as I am allowed to do with fallible docuмents (especially those which contract infallible ones).