I said that wearing those clothes is a mortal sin. I said that if someone dies by accident, and has one mortal sin on their soul, that they will not be saved.
One of the conditions which make a sin mortal: it must be committed with full knowledge, both of the sin and of the gravity of the offense.
That simple quote has been used for over 50 years to excuse everybody from practically every sin (except Adolph Hitler of course).
That whatever is done through ignorance must not be considered a sin, is hereby condemned as error. Pope Innocent II
It does not suffice to say: "If I had known that such a thing were forbidden by the law of God, I would have conformed." St. Paul, in persecuting Christians, previous to his conversion, did not think that he was committing evil; the same may be said of Jews who persecuted and crucified Christ. And yet, Saint Paul and these Jews were not innocent and excusable. If they had died in that state, they would never have been saved. Hence, St. Paul, despite the good faith he acted on, acknowledges that he was at that time a blasphemer, and unjust: a persecutor of the Church. If, therefore, a person be ignorant of what is commanded¬ or forbidden because he has not studied the law of God, his ignorance does not excuse him from sin. St. Alphonsus Maria Liguori
If ignorance is not a sin, then Saul did not sin when he persecuted the Church, because he surely did this in ignorance. Therefore, he should not have said: “I obtained the mercy of God” (I Tim 1:13), but rather “I received my reward”. St. Bernard
Those who keep their eyes shut cannot see. God made you without your knowledge, but He does not justify you without your willing it. Refusal to hear the truth leads to sin, and that sin itself is punishment for the preceding sin. Every sinner is inexcusable whether he knows it or not. For ignorance itself, in those who do not want to know, is without doubt a sin; and, in those unable to know, is the penalty of sin. In neither case, then, is there a just excuse, but in both cases there is a just condemnation. St. Augustine
It follows that ignorance has the nature of mortal sin on account of either a preceding negligence, or the consequent result; and, for this reason, ignorance is reckoned one of the general causes of sin. All sin proceeds from ignorance. St. Thomas Aquinas
Unbelief has the character of guilt from resisting the faith, rather than from mere absence of the faith. St Thomas Aquinas
Innumerable souls are miserably lost through ignorance of religion the source of every other calamity. St Frances Xavier Cabrini
Reflect on the ruin of souls wrought by this single cause: ignorance of truths which must be known by all men alike in order that they may attain eternal salvation. This we solemnly affirm: the majority of those condemned to eternal Punishment fall into everlasting misfortune through ignorance of the Mysteries of the faith, which must necessarily be known and believed by all who belong to the Elect. Pope St. Pius X
The excuse of ignorance is denied those who know the commandments of God, but neither will those who do not know be without punishment. "For, as many as have sinned outside the law shall also perish outside the law" (Romans 2:12). Without faith in Christ, no man can be delivered; therefore, they will be judged in such a way that they perish. "The ser¬vant who does not know his Lord's will, and who commits things worthy of stripes, shall be beaten with few stripes , whereas, the servant who knows his Lord's will, shall be beaten with many stripes" (Lk. 12:47 48). Observe here that it is a more serious matter for a man to sin with knowledge than in ignorance. And yet, we must not take refuge on this account to shades of ignorance, to find our excuse therein. Even ignorance which belongs to them who are, as it were, simply ignorant does not excuse any¬ one so as to exempt him from eternal fire, even were his failure to believe the result of not having heard at all what he should believe. It was not said with¬ out reason: "Pour out Thy wrath upon the nations who have not known Thee" (Psalm 78:6), and "He shall come from Heaven in a flame of fire to take vengeance on those who do not know God" (Thess.11 1:7 8) St. Augustine
Perhaps he who asserts that a person cannot sin through ignorance never prays for his ignorances, but laughs at the prophet who prays: "O Lord, remember not the sins of my ignorances!" (Ps. 24:7). Perhaps he even reproves God, Who requires satisfaction for the sin of ignorance, for in Leviticus He speaks of "sin through ignorance." If ignorance were never a sin, why is it that the High Priest entered the second tabernacle with blood, which he offered "for his own ignorance and for the ignorance of the people" (Heb. 9:7)? If one never sins through ignorance, then what do we hold against those who killed the Apostles, since they truly did not know that to kill them was evil but rather thought that they were "doing a service to God" (Jn. 16:2). Thus, also, Our Sa-vior prayed in vain on the cross for those who cruci¬fied Him since, as He Himself testifies, they were ig¬norant of what they were doing (Lk. 23:34) and thus they did not sin at all! Neither should anyone suspect that the Apostle could have lied when he said: "For, if they had known it, they would never have crucified the Lord" (I Corinth. 2:8). Is it not sufficiently clear, from these passages, in what great ignorance the man lies who does not know that one can some¬ times sin through ignorance? St. Bernard