St. Thomas Aquinas: “Invincible Ignorance is a Punishment for Sin”

“Invincible ignorance is a punishment for sin.” - St. Thomas Aquinas (De Infid. q. x., art. 1.)

 "Still we answer the Semipelagians, and say, that infidels who arrive at the use of reason, and are not converted to the Faith, cannot be excused, because though they do not receive sufficient proximate Grace, still they are not deprived of remote Grace, as a means of becoming converted. But what is this remote Grace? St. Thomas explains it, when he says, that if any one was brought up in the wilds, or even among brute beasts, and if he followed the law of natural reason, to desire what is good, and to avoid what is wicked, we should certainly believe either that God, by an internal inspiration, would reveal to him what he should believe, or would send some one to preach the Faith to him, as he sent Peter to Cornelius. Thus, then, according to the Angelic Doctor, God, at least remotely, gives to the infidels, who have the use of reason, sufficient Grace to obtain salvation, and this Grace consists in a certain instruction of the mind, and in a movement of the will, to observe the natural law; and if the infidel co-operates with this movement, observing the precepts of the law of nature, and abstaining from grievous sins, he will certainly receive, through the merits of Jesus Christ, the Grace proximately sufficient to embrace the Faith, and save his soul." - St. Alphonsus Liguori, A History of Heresies and their Refutation

“Brothers, you must know that the most ancient belief is the Law of God, and that we all bear it written in our hearts; that it can be learned without any teacher, and that it suffices to have the light of reason in order to know all of the precepts of that Law. That is why even the barbarians hid when they committed sin, because they knew they were doing wrong; and they are damned for not having observed the natural law written in their heart: for had they observed it, God would have made a miracle rather than let them be damned; He would have sent them someone to teach them and would have given them other aids, of which they made themselves unworthy by not living in conformity with the inspirations of their own conscience.” - St. Leonard of Port Maurice, Sermon on the Fewness of the Saved