The Dimond brothers pointed out Fr. Michael Müller as a good source for understanding what Pope Pius IX meant, when he spoke of invincible ignorance. I thought I would share these passages on the subject. Does anyone know of clerics who think and teach the same as Fr. Müller here?
From "The Church and Her Enemies", 1880, p. 284:
Here it may be asked: Are all those who are out of the Church equally guilty in the sight of God? We answer: No; some are more guilty than others. It cannot be considered the personal fault, however great and terrible the misfortune, of any individual of the children of Adam that our first parents sinned. So it is not the fault of those who were born and educated in any of the errors or negations of Protestantism, in its hundred various forms. Involuntary error is a misfortune to be pitied, a calamity to be deplored. Only when entered into, or persisted in, against light and knowledge, can it be considered a sin, or other than a sin of ignorance. There are persons who sometimes commit actions which, in themselves, are very wrong, but are not punishable in the sight of God, because they do not proceed from wilful malice, as those who commit them are not aware in the least that by such actions God is offended. So there may be persons who live in infidelity or heresy without being in the least aware of it. Now such inculpable ignorance will, of course, not save them; but, if they fear God, and live up to their conscience, God, in his infinite mercy, will furnish them with the necessary means of salvation, even so as to send, if needed, an angel to instruct them in the Catholic faith, rather than let them perish through inculpable ignorance.
But there are others who are guilty in the sight of God. They are those who know the Catholic Church to be the only true Church, but do not embrace her faith, as also those who could know her, if they would candidly search, but who, through indifference, and other culpable motives, neglect to do so.