Objection 1. It would seem that the will is good when it abides by erring reason. For just as the will, when at variance with the reason, tends to that which reason judges to be evil; so, when in accord with reason, it tends to what reason judges to be good. But the will is evil when it is at variance with reason, even when erring. Therefore even when it abides by erring reason, the will is good.
Objection 2. Further, the will is always good, when it abides by the commandment of God and the eternal law. But the eternal law and God's commandment are proposed to us by the apprehension of the reason, even when it errs. Therefore the will is good, even when it abides by erring reason.
Objection 3. Further, the will is evil when it is at variance with erring reason. If, therefore, the will is evil also when it abides by erring reason, it seems that the will is always evil when in conjunction with erring reason: so that in such a case a man would be in a dilemma, and, of necessity, would sin: which is unreasonable. Therefore the will is good when it abides by erring reason.
On the contrary, The will of those who slew the apostles was evil. And yet it was in accord with the erring reason, according to John 16:2: "The hour cometh, that whosoever killeth you, will think that he doth a service to God." Therefore the will can be evil, when it abides by erring reason.
I answer that, Whereas the previous question is the same as inquiring "whether an erring conscience binds"; so this question is the same as inquiring "whether an erring conscience excuses." Now this question depends on what has been said above about ignorance. For it was said (6, 8) that ignorance sometimes causes an act to be involuntary, and sometimes not. And since moral good and evil consist in action in so far as it is voluntary, as was stated above (Article 2); it is evident that when ignorance causes an act to be involuntary, it takes away the character of moral good and evil; but not, when it does not cause the act to be involuntary. Again, it has been stated above (Question 6, Article 8) that when ignorance is in any way willed, either directly or indirectly, it does not cause the act to be involuntary. And I call that ignorance "directly" voluntary, to which the act of the will tends: and that, "indirectly" voluntary, which is due to negligence, by reason of a man not wishing to know what he ought to know, as stated above (Question 6, Article 8).
If then reason or conscience err with an error that is voluntary, either directly, or through negligence, so that one errs about what one ought to know; then such an error of reason or conscience does not excuse the will, that abides by that erring reason or conscience, from being evil. But if the error arise from ignorance of some circumstance, and without any negligence, so that it cause the act to be involuntary, then that error of reason or conscience excuses the will, that abides by that erring reason, from being evil. For instance, if erring reason tell a man that he should go to another man's wife, the will that abides by that erring reason is evil; since this error arises from ignorance of the Divine Law, which he is bound to know. But if a man's reason, errs in mistaking another for his wife, and if he wish to give her her right when she asks for it, his will is excused from being evil: because this error arises from ignorance of a circumstance, which ignorance excuses, and causes the act to be involuntary.
Reply to Objection 1. As Dionysius says (Div. Nom. iv), "good results from the entire cause, evil from each particular defect." Consequently in order that the thing to which the will tends be called evil, it suffices, either that it be evil in itself, or that it be apprehended as evil. But in order for it to be good, it must be good in both ways.
Reply to Objection 2. The eternal law cannot err, but human reason can. Consequently the will that abides by human reason, is not always right, nor is it always in accord with the eternal law.
Reply to Objection 3. Just as in syllogistic arguments, granted one absurdity, others must needs follow; so in moral matters, given one absurdity, others must follow too. Thus suppose a man to seek vainglory, he will sin, whether he does his duty for vainglory or whether he omit to do it. Nor is he in a dilemma about the matter: because he can put aside his evil intention. In like manner, suppose a man's reason or conscience to err through inexcusable ignorance, then evil must needs result in the will. Nor is this man in a dilemma: because he can lay aside his error, since his ignorance is vincible and voluntary.
Source: Summa Theologica (New Advent)