The term "invincible ignorance" was used to refer to the state of persons (such as pagans and infants) who are ignorant of the Christian message because they have not yet had an opportunity to hear it. Aquinas, for instance, uses it in the Summa, when and how the term was taken by liberals to refer to the very different state of persons who pigheadedly refuse to attend to evidence remains unclear to me. The important point is that St. Thomas and St. Alphonsus Ligouri rejected it as a way for non-Cathoics to be saved.
No one is more invincible ignorant than an infant, and unless they are baptized, they are not saved.
St. Alphonsus Liguori, Sermons (c. +1760): “How many are born among the pagans, among the Jews, among the Mohometans and heretics, and all are lost.”
St. Alphonsus: “If you are ignorant of the truths of the faith, you are obliged to learn them. Every Christian is bound to learn the Creed, the Our Father, and the Hail Mary under pain of mortal sin. Many have no idea of the Most Holy Trinity, the Incarnation, mortal sin, Judgment, Paradise, Hell, or Eternity; and this deplorable ignorance damns them.”
St. Alphonsus, Preparation For Death, (c. +1760): “How thankful we ought to be to Jesus Christ for the gift of faith! What would have become of us if we had been born in Asia, Africa, America, or in the midst of heretics and schismatics? He who does not believe is lost. This, then, was the first and greatest grace bestowed on us: our calling to the true faith. O Savior of the world, what would become of us if Thou hadst not enlightened us? We would have been like our fathers of old, who adored animals and blocks of stone and wood: and thus we would have all perished.”
Fr. Michael Muller, C.SS.R., The Catholic Dogma, pp. 217-218, 1888: “Inculpable or invincible ignorance has never been and will never be a means of salvation. To be saved, it is necessary to be justified, or to be in the state of grace. In order to obtain sanctifying grace, it is necessary to have the proper dispositions for justification; that is, true divine faith in at least the necessary truths of salvation, confident hope in the divine Savior, sincere sorrow for sin, together with the firm purpose of doing all that God has commanded, etc. Now, these supernatural acts of faith, hope, charity, contrition, etc., which prepare the soul for receiving sanctifying grace, can never be supplied by invincible ignorance; and if invincible ignorance cannot supply the preparation for receiving sanctifying grace, much less can it bestow sanctifying grace itself. ‘Invincible ignorance,’ says St. Thomas, ‘is a punishment for sin.’ (De, Infid. Q. x., art. 1).
“It is, then, a curse, but not a blessing or a means of salvation… Hence Pius IX said ‘that, were a man to be invincibly ignorant of the true religion, such invincible ignorance would not be sinful before God; that, if such a person should observe the precepts of the Natural Law and do the will of God to the best of his knowledge, God, in his infinite mercy, may enlighten him so as to obtain eternal life; for, the Lord, who knows the heart and the thoughts of man will, in his infinite goodness, not suffer anyone to be lost forever without his own fault.’ Almighty God, who is just condemns no one without his fault, puts, therefore, such souls as are in invincible ignorance of the truths of salvation, in the way of salvation, either by natural or supernatural means.”