Invincible ignorance, whether of the law or of the fact, is always a valid excuse and excludes sin. The evident reason is that neither this state nor the act resulting therefrom is voluntary. It is undeniable that a man cannot be invincibly ignorant of thenatural law, so far as its first principles are concerned, and the inferences easily drawn therefrom. This, however, according to the teaching of St. Thomas, is not true of those remoter conclusions, which are deducible only by a process of laborious and sometimes intricate reasoning. Of these a person may be invincibly ignorant. Even when the invincible ignorance is concomitant, it prevents the act which it accompanies from being regarded as sinful. The perverse temper of soul, which in this case is supposed, retains, of course, such malice as it had. Vincible ignorance, being in some way voluntary, does not permit a man to escape responsibility for the moral deformity of his deeds; he is held to be guilty and in general the more guilty in proportion as his ignorance is more voluntary. Hence, the essential thing to remember is that the guilt of an act performed or omitted in vincible ignorance is not to be measured by the intrinsic malice of the thing done or omitted so much as by the degree ofnegligence discernible in the act.
It must not be forgotten that, although vincible ignorance leaves the culpability of a person intact, still it does make the act lessvoluntary than if it were done with full knowledge. This holds good except perhaps with regard to the sort of ignorance termed affected. Here theologians are not agreed as to whether it increases or diminishes a man's moral liability. The solution is possibly to be had from a consideration of the motive which influences one in choosing purposely to be ignorant. For instance, a man who would refuse to learn the doctrines of the Church from a fear that he would thus find himself compelled to embrace them would certainly be in a bad plight. Still he would be less guilty than the man whose neglect to know the teachings of the Church wasinspired by sheer scorn of her authority. Invincible ignorance, whether of the law or fact, exempts one from the penalty which may have been provided by positive legislation. Even vincible ignorance, either of the law or fact, which is not crass, excuses one from the punishment. Mere lack of knowledge of the sanction does not free one from the penalty except in cases ofcensures. It is true then that any sort of ignorance which is not itself grievously sinful excuses, because for the incurring ofcensures contumacy is required. Vincible and consequent ignorance about the duties of our state of life or the truths of faithnecessary for salvation is, of course, sinful. Ignorance of the nature or effects of an act does not make it invalid if everything else requisite for its validity be present. For instance, one who knows nothing of the efficacy of baptism validly baptizes, provided that he employs the matter and form and has the intention of doing what the Church does.
http://www.newadvent.org/cathen/07648a.htm
more later
http://www.newadvent.org/cathen/07648a.htm
more later