Objection 1. It would seem that ignorance excuses from sin altogether. For as Augustine says (Retract. i, 9), every sin is voluntary. Now ignorance causes involuntariness, as stated above (Question 6, Article 8). Therefore ignorance excuses from sin altogether.
Objection 2. Further, that which is done beside the intention, is done accidentally. Now the intention cannot be about what is unknown. Therefore what a man does through ignorance is accidental in human acts. But what is accidental does not give the species. Therefore nothing that is done through ignorance in human acts, should be deemed sinful or virtuous.
Objection 3. Further, man is the subject of virtue and sin, inasmuch as he is partaker of reason. Now ignorance excludes knowledge which perfects the reason. Therefore ignorance excuses from sin altogether.
On the contrary, Augustine says (De Lib. Arb. iii, 18) that "some things done through ignorance are rightly reproved." Now those things alone are rightly reproved which are sins. Therefore some things done through ignorance are sins. Therefore ignorance does not altogether excuse from sin.
I answer that, Ignorance, by its very nature, renders the act which it causes involuntary. Now it has already been stated (1,2) that ignorance is said to cause the act which the contrary knowledge would have prevented; so that this act, if knowledge were to hand, would be contrary to the will, which is the meaning of the word involuntary. If, however, the knowledge, which is removed by ignorance, would not have prevented the act, on account of the inclination of the will thereto, the lack of this knowledge does not make that man unwilling, but not willing, as stated in Ethic. iii, 1: and such like ignorance which is not the cause of the sinful act, as already stated, since it does not make the act to be involuntary, does not excuse from sin. The same applies to any ignorance that does not cause, but follows or accompanies the sinful act.
On the other hand, ignorance which is the cause of the act, since it makes it to be involuntary, of its very nature excuses from sin, because voluntariness is essential to sin. But it may fail to excuse altogether from sin, and this for two reasons. First, on the part of the thing itself which is not known. For ignorance excuses from sin, in so far as something is not known to be a sin. Now it may happen that a person ignores some circumstance of a sin, the knowledge of which circumstance would prevent him from sinning, whether it belong to the substance of the sin, or not; and nevertheless his knowledge is sufficient for him to be aware that the act is sinful; for instance, if a man strike someone, knowing that it is a man (which suffices for it to be sinful) and yet be ignorant of the fact that it is his father, (which is a circumstance constituting another species of sin); or, suppose that he is unaware that this man will defend himself and strike him back, and that if he had known this, he would not have struck him (which does not affect the sinfulness of the act). Wherefore, though this man sins through ignorance, yet he is not altogether excused, because, not withstanding, he has knowledge of the sin. Secondly, this may happen on the part of the ignorance itself, because, to wit, this ignorance is voluntary, either directly, as when a man wishes of set purpose to be ignorant of certain things that he may sin the more freely; or indirectly, as when a man, through stress of work or other occupations, neglects to acquire the knowledge which would restrain him from sin. For such like negligence renders the ignorance itself voluntary and sinful, provided it be about matters one is bound and able to know. Consequently this ignorance does not altogether excuse from sin. If, however, the ignorance be such as to be entirely involuntary, either through being invincible, or through being of matters one is not bound to know, then such like ignorance excuses from sin altogether.
Reply to Objection 1. Not every ignorance causes involuntariness, as stated above (Question 6, Article 8). Hence not every ignorance excuses from sin altogether.
Reply to Objection 2. So far as voluntariness remains in the ignorant person, the intention of sin remains in him: so that, in this respect, his sin is not accidental.
Reply to Objection 3. If the ignorance be such as to exclude the use of reason entirely, it excuses from sin altogether, as is the case with madmen and imbeciles: but such is not always the ignorance that causes the sin; and so it does not always excuse from sin altogether.
Source: Summa Theologica (New Advent)