Traditionally, this subject was called “the culpably erroneous conscience” or “vincible ignorance”—a lack of knowledge one could overcome. By my analysis there are three distinct situations: (1) conscience which is quiescent or not operating on the level of moral truth as it could and should; (2) conscience which delivers an erroneous judgment which one could and should question, but does not; (3) conscience which delivers an erroneous judgment which one is psychologically incapable of questioning at present due to one’s own prior, grave fault.
In the first case, one is responsible morally only for one’s prior wrong acts or omissions which block the operation of mature conscience. Here and now there is no genuine judgment of conscience which is ignored or violated.
In the second case, one’s moral state is rather similar to that of a doubtful conscience, in the sense that one ought to investigate before acting. However, the duty to investigate is not so clear, because the unsatisfactory state of one’s conscience is not directly before one’s mind.
For example, a businessman once heard that in many cases one is not morally obliged to pay all of the taxes which would be required by a strict interpretation of the law. He has cut corners for some time, but not investigated whether his corner-cutting falls within the boundaries of the moral teaching of the Church. Now and then it occurs to him that he ought to think this matter through, but when it comes time to prepare taxes he is always busy and inclined to assume that the vague moral formation he once received is adequate to cover what he is doing. This is a typical case of vincible ignorance; to the extent that the judgments the businessman makes are false, he also is in culpable error.
This state of mind can have two quite different moral backgrounds. In one, there earlier was a mortal sin—for example, a serious act of fraud—which was never repented. In such a case, the conveniently ignorant and probably erring conscience belongs to the sinful pattern of life. But in another moral background, there was no earlier, unrepented mortal sin, and the man never clearly faced and violated a judgment of conscience in a grave matter. In this latter case, until he becomes aware that he has a grave obligation to resolve the moral issue of his tax evasion, the businessman has not committed a mortal sin, even though his ignorance might be culpable and the evasion of taxes grave matter.
The third case—that of a person at present psychologically unable to uncover the error of conscience initiated by a prior act of his or her own—likewise divides into two morally different subtypes.
One of these is the bad faith of a person whose false conscience is generated by a persistent state of mortal sin. “For every one who does evil hates the light, and does not come to the light, lest his deeds should be exposed” (Jn 3.20). Truthful preaching and teaching arouse resentment in such persons, because they are forced to encounter the truth they could not bring to the surface for themselves, and they will not consider submitting to the truth when they are forced to face it (see Jn 12.37–43).
Quite different is the error of the simple person who tries to avoid and overcome grave sin, who perhaps is not well instructed or very intelligent, but who by many venial sins of commission and omission has a conscience filled with unsuspected misinformation. Such a person might do things gravely evil in themselves without realizing their character. The error in conscience is culpable and subjectively correctable, but there is no mortal sin.
Perhaps, also, one might include here the erroneous consciences of generally upright people who accept without question the moral errors of their times, although they should be sophisticated enough to see through them. Those who burned heretics ought to have known better. Why didn’t they? Perhaps because of many venial sins, some serious, of prejudice, vindictiveness, anxiety about the faith (a sin against hope), arrogant overconfidence in the system of which they were functionaries, insufficient diligence to think the matter through, and so on.