Dear all,
In our many debates on baptism-of-desire, i've always argued that 'invincible ignorance' is a myth and cannot exist.  I've said many times that no one can be ignorant because Scripture tells us in John 1 that Christ 'enlightens every man who comes into this world', so every man is born with a natural knowledge of God, and a conscience, a guardian angel and the natural law written on his heart.  Because God gives these built-in actual graces to every human being, i've argued that it's only logical that if a man cooperates with his guardian angel/conscience, his knowledge of natural law good/evil and his knowledge that God exists (have you ever met a 4 year old atheist?), then God will lead him to the truth.  Therefore, invincible ignorance is impossible.
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Even St Thomas says that ignorance does exist, and is a punishment for sin.  But can invincible ignorance exist, in spite of all of this?  I've always said yes, but I am wrong.  Here's why.
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IGNORANCE - (from the latin word, "ignorare").  To not know even though necessary information is present, because that information has been willfully refused or disregarded.
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NESCIENCE - (from the latin word, "nesciere").  To not know because knowledge was absent or unattainable.
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Conclusion - So, yes, 'invincible ignorance' does exist.  I was wrong.  I have been using the wrong word.  Thousands of people have been using the word word.  Maybe it is due to improper english translations from the latin.  "Invincible Ignorance" does surely exist, because many, many people willfully refuse and disregard the truth.  This is why St Thomas said that ignorance is a punishment for sin, because prior sin darkens the mind, and the consequent pride, stubbornness and disregard for spiritual things, makes one reject and disregard truth, when it is presented.  So the person stays ignorance of truth, but willingly so.
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But nescience does not, and cannot exist.  No one can ever claim that knowledge of truth, or knowledge of good/evil, or knowledge of God is not present or possible to attain.  No human being can ever blame God for not giving him the opportunity to avoid hell (i.e. to at least attain Limbo), because everyone can (and has the duty to) follow the natural law, which is written on all men's hearts (as John 1 tells us, this is part of Christ's enlightenment).
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From the very instant that every man is created, he is given an immortal soul, which is created in the image of God.  Thus, no man can complain that he does not recognize Truth, or did not know his Creator, or did not understand the natural law, because how can a man not know or understand a part of himself?  All men have the ability to instantly recognize Truth because our soul's life-giving essence is made from Eternal Truth.  
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Knowledge of God and Truth is always attainable because God is omnipresent and our soul is attracted to God infinitely more than a bug is attracted to light.  Only sin, which wounds the soul and darkens the intellect can put an obstacle between us and God.  If someone is not saved, it is due to ignorance not nescience.