I am a diocesan priest in the parish of Cyrill and Method in the Archdiocese of Vienna, and a teacher at the ‘It is [sic] can no longer simply be said that all those in any “irregular” situation are living in a state of mortal sin and are deprived of sanctifying grace. More is involved here than mere ignorance of the rule. A subject may know full well the rule, yet have great difficulty in understanding “its inherent values”, or be in a concrete situation which does not allow him or her to act differently and decide otherwise without further sin.’ (AL 301)
The theologians claim that this statement understood as meaning that a Catholic believer "can have full knowledge of a divine law and voluntarily choose to break it in a serious matter, but not be in a state of mortal sin as a result of this action", is heretical.
Amoris Laetitia seems to suggest, at least with the second part of the hypothetical "A subject may know full well the true, yet… be in a concrete situation which does not allow him or her to act differently and decide otherwise without further sin" that an individual may, due to being obligated in conscience to act a certain way ("does not allow him or her to act differently… without further sin"), not bear grave guilt in the violation of the "rule" that puts them in an "irregular" situation.
In this hypothetical, Amoris Laetitia is envisioning somewhat who is caught between two (perceived) obligations: the obligation of the rule in question, and the obligation of another rule that applies in this particular circumstance. For example, some one who married for a few years, without children, was divorced by the first spouse, has civilly remarried in ignorance of the fact that divorce cannot dissolve a valid marriage, been living in this civil marriage for many years and had children with the second partner, may come to learn of the indissolubility of marriage and the consequent obligation of fidelity to the first spouse, yet at the same time believe themselves bound by the grave obligation of justice to their second partner (including sexual giving of self to the partner). Such a person, caught between two apparent divine obligations, may discern in conscience that their greater obligation is to the current partner; the argument of Amoris Laetitia implies that such a person does not thereby necessarily commit a mortal sin.
One might quibble about whether such a person really knows "full well" the rule, but that is more an argument about words than a substantial argument about the matter in question.
The two major points one might raise are: is it possible, without grave fault, to believe that one divine law obliges one to break another divine law? In general St. Thomas Aquinas held that ignorance regarding divine law was culpable, and so, in speaking about the case of a perplexed conscience, which obliges a person to do something contrary to divine law, he argues that a person, so long as they are in that ignorance, sins by disobeying their conscience, since they do not follow the divine rule as it is presented to them by their conscience, and likewise sines by following their conscience, since they do not follow the divine rule that they should have know. He does make exceptions for insane persons, who are not capable of any moral action, but it is not clear whether he would make any further exceptions. However, considering the various external factors that lead someone to accept something as a moral obligation, it seems necessary to admit that in some cases, ignorance regarding divine laws may be not gravely culpable. If this is true, someone who erroneously believes that he is bound by a "higher" (as far as he perceives it) divine law to disobey another divine law, would be inculpable. One may not be obliged, by catholic Faith, to believe that there are such cases. But it is not in the least heretical to hold that there are; and indeed, this very statement of Amoris Laetitia provides a reason, based on church authority, to hold that there are, even if a small reason considered in itself.
The first part of the hypothetical "may… have great difficulty in understanding 'its inherent values'", I find harder to interpret, and potentially more problematic. But whatever exactly is meant by understanding "its inherent values", it seems to imply at any rate not really having "full knowledge" of a divine law in a morally relevant sense. I may come back to this, if I think I have something enlightening to say about a relationship between "knowing a rule" and "understanding its inherent values".
I think this Disputed Question Q17 is pertinent:
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A false conscience which is mistaken in things which are intrinsically evil commands something which is contrary to the law of God. Nevertheless, it says that what it commands is the law of God. Accordingly, one who acts against such a conscience becomes a kind of transgressor of the law of God, although *one Who follows such a conscience and acts according to it acts against the law of God and sins mortally.* For there was sin in the error itself, since it happened because of ignorance of that which one should have known.
One whose conscience tells him to commit fornication is not completely perplexed, because *he can do something by which he can avoid sin, namely, change the false conscience.* But he is perplexed to some degree, that is, as long as the false conscience remains. And there is no difficulty in saying that, if some condition is presupposed, it is impossible for a man to avoid sin; just as, if we presuppose the intention of vainglory, one who is required to give alms cannot avoid sin. For, if he gives alms, because of such an intention, he sins; but, if he does not give alms, he violates the law.
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While this does allow *conditionally* where someone could not avoid sin, ie, so long as the condition of their false conscience remains, of course this condition is not true as such, but only the product of their false or perplexed conscience.
From this, I see no opening for inculpability unless the conscience is invincibly ignorant, which is not to be assumed in violations of the Natural Law (in this case, the 6th Commandment) which is written on the heart.
In the event its of interest, please see my Apologia for the orthodoxy of Amoris Laetitia (https://reducedculpability.blog/2017/01/19/amoris-laetitia-an-apologia-for-its-orthodoxy/).
Are you sure that St. Thomas taught that a person cannot be invincibly ignorant of divine law? I've read theologians who have said that a person can be invincibly ignorant of Christ's call for all men to enter the Church, for example, and I would presume that that is of divine law. Another example is confession – that we must submit our sins to the power of the keys via absolution, and I've read theologians describe this as being of divine law as well. Yet, they generally admitted that one can be saved without it through perfect contrition among non-Catholics. So perhaps what's meant here is natural law? That there are certain principles of natural law that we cannot claim ignorance of (the first principles, like doing good and avoiding evil), whereas its secondary principles can be obscured within the hearts of man (whether they can be invincibly ignorant of them, and on what matters specifically, I'm unsure to be quite honest with you).
If you're up to it, maybe it would be a good idea to do a quick check on the moral theology manuals, where they deal with the subject of a perplexed conscience and the conditions for mortal sin, ignorance of the law etc. In such matters, I think they can provide much light. And if I remember correctly, they admit that when it comes to a perplexed conscience, a person is to wait and try to obtain information, but if that's not possible, they are to choose the less likely evil, and by doing so they are inculpable, for God does not command the impossible.
I just noticed your comment waiting for approval, don't have time to give a detailed answer, which would be a bit more involved, but just want to cite several texts, fairly representative of Aquinas's position:
If then reason or conscience errs with an error that is voluntary, either directly or through negligence, so that one errs about something that one is bound to know, then such an error of reason or conscience does not excuse from being evil the will that is in accordance with that erring reason or conscience. But if the error arises from ignorance of some circumstance, and without any negligence, then it causes the act to be involuntary, and that error of reason or conscience excuses from being evil the will that is in accordance with that erring reason. For example, if erring reason tells a man that he should have sexual intercourse with another man's wife, the will that is in accordance with that erring reason is evil, since this error arises from ignorance of the divine law (ex ignorantia legis Dei, quam scire tenetur), which he is bound to know. But if a man's reason errs in mistaking another woman for his wife, and if he wills to give her her right when she asks for it, his will is excused from being evil, since this error arises from ignorance of a circumstance, which ignorance excuses, and causes the act to be involuntary. (ST I-II, q. 19, a. 6)
In this particular text, the Latin text could theoretically also be translated "from ignorance of a divine law that he is bound to know (one that is also of natural law)". However, Aquinas applies the same principle to a heretic obliged by his conscience to teach things contrary to the catholic faith. While the specific obligation to accept whatever God teaches may belong to natural law, the specific content of revelation certainly does not belong to natural law.
2. Sometimes an erring conscience tells a man to do what is against God's law; e.g., the erring conscience of a heretic tells him to preach contrary to catholic faith. But by acting contrary to God's law, he sins mortally. If, therefore, my acting against his conscience he would sin mortally, it would follow that both would be a mortal sin, to preach contrary to God's law or not to preach. And thus he would be perplexed… Therefore an erring conscience does not oblige.
To the second it should be said that if conscience tells someone to do what is contrary to God's law, he sins if he does not do it; and similarly he sins if he does; for ignorance of the law does not excuse from sin, unless it is invincible ignorance, as in the case of mentally insane persons, ignorance altogether excuses. It does not, however, follow that he is perplexed simply speaking, but only in a certian respect. For he can put aside the erring conscience, and then he does not sin by acting according to God's law. (Quodlibetal 3, q. 12, a. 2)