Objection 1. It would seem that ignorance is not a sin. For sin is "a word, deed or desire contrary to God's law," as stated above (Question 71, Article 5). Now ignorance does not denote an act, either internal or external. Therefore ignorance is not a sin.
Objection 2. Further, sin is more directly opposed to grace than to knowledge. Now privation of grace is not a sin, but a punishment resulting from sin. Therefore ignorance which is privation of knowledge is not a sin.
Objection 3. Further, if ignorance is a sin, this can only be in so far as it is voluntary. But if ignorance is a sin, through being voluntary, it seems that the sin will consist in the act itself of the will, rather than in the ignorance. Therefore the ignorance will not be a sin, but rather a result of sin.
Objection 4. Further, every sin is taken away by repentance, nor does any sin, except only original sin, pass as to guilt, yet remain in act. Now ignorance is not removed by repentance, but remains in act, all its guilt being removed by repentance. Therefore ignorance is not a sin, unless perchance it be original sin.
Objection 5. Further, if ignorance be a sin, then a man will be sinning, as long as he remains in ignorance. But ignorance is continual in the one who is ignorant. Therefore a person in ignorance would be continually sinning, which is clearly false, else ignorance would be a most grievous sin. Therefore ignorance is not a sin.
On the contrary, Nothing but sin deserves punishment. But ignorance deserves punishment, according to 1 Corinthians 14:38: "If any man know not, he shall not be known." Therefore ignorance is a sin.
I answer that, Ignorance differs from nescience, in that nescience denotes mere absence of knowledge; wherefore whoever lacks knowledge about anything, can be said to be nescient about it: in which sense Dionysius puts nescience in the angels (Coel. Hier. vii). On the other hand, ignorance denotes privation of knowledge, i.e. lack of knowledge of those things that one has a natural aptitude to know. Some of these we are under an obligation to know, those, to wit, without the knowledge of which we are unable to accomplish a due act rightly. Wherefore all are bound in common to know the articles of faith, and the universal principles of right, and each individual is bound to know matters regarding his duty or state. Meanwhile there are other things which a man may have a natural aptitude to know, yet he is not bound to know them, such as the geometrical theorems, and contingent particulars, except in some individual case. Now it is evident that whoever neglects to have or do what he ought to have or do, commits a sin of omission. Wherefore through negligence, ignorance of what one is bound to know, is a sin; whereas it is not imputed as a sin to man, if he fails to know what he is unable to know. Consequently ignorance of such like things is called "invincible," because it cannot be overcome by study. For this reason such like ignorance, not being voluntary, since it is not in our power to be rid of it, is not a sin: wherefore it is evident that no invincible ignorance is a sin. On the other hand, vincible ignorance is a sin, if it be about matters one is bound to know; but not, if it be about things one is not bound to know.
Reply to Objection 1. As stated above (71, 6, ad 1), when we say that sin is a "word, deed or desire," we include the opposite negations, by reason of which omissions have the character of sin; so that negligence, in as much as ignorance is a sin, is comprised in the above definition of sin; in so far as one omits to say what one ought, or to do what one ought, or to desire what one ought, in order to acquire the knowledge which we ought to have.
Reply to Objection 2. Although privation of grace is not a sin in itself, yet by reason of negligence in preparing oneself for grace, it may have the character of sin, even as ignorance; nevertheless even here there is a difference, since man can acquire knowledge by his acts, whereas grace is not acquired by acts, but by God's favor.
Reply to Objection 3. Just as in a sin of transgression, the sin consists not only in the act of the will, but also in the act willed, which is commanded by the will; so in a sin of omission not only the act of the will is a sin, but also the omission, in so far as it is in some way voluntary; and accordingly, the neglect to know, or even lack of consideration is a sin.
Reply to Objection 4. Although when the guilt has passed away through repentance, the ignorance remains, according as it is a privation of knowledge, nevertheless the negligence does not remain, by reason of which the ignorance is said to be a sin.
Reply to Objection 5. Just as in other sins of omission, man sins actually only at the time at which the affirmative precept is binding, so is it with the sin of ignorance. For the ignorant man sins actually indeed, not continually, but only at the time for acquiring the knowledge that he ought to have.
Source: Summa Theologica (New Advent)