592. An erroneous conscience may apprehend something not wrong as wrong, but in an indeterminate manner.
(a) If the species of evil is not determinate before the conscience, but an indifferent act is thought to be sinful without any definite species of sin being thought of, he who acts against such a conscience seems to commit a sin of disobedience. Example: A person who thinks that smoking is a sin, of what kind he does not know, must have at least vaguely the opinion that it is forbidden by the divine law; and hence, if he smokes, he is guilty of disobedience.
(b) If the gravity of the putative sin is not determinate before the conscience, but an act is thought to be sinful without the degree of sinfulness being at all known or thought of, he who acts against such a conscience commits a mortal or a venial sin according to his own disposition with respect to sin. If he is so attached to the sin he apprehends that he intends to commit it, whether it be great or small, he is guilty of mortal sin, at least in so far as he exposes himself to it. But if he is habitually resolved not to commit grave sin, it can be presumed that he would not do that which he apprehends as sinful, if he thought it was a grave offense, Example: If a person erroneously thinks that it is a sin to read a certain book, and then reads it without adverting at all to the gravity of the sin he apprehends, his greater or less guilt will have to be judged by his character. If he is so conscientious that he would stop reading at once if he feared the book was seriously harmful, he sins only venially; but if he knows that he is lax and is yet resolved to read the book at all costs, it seems that he is guilty of grave sin.
Art. 2: A GOOD CONSCIENCE
(_Summa Theologica_, I-II, q. 19, aa. 5, 6.)
593. As was explained in the previous article, conscience is not a proper guide unless it is good. In this article we shall speak of the good conscience and of its opposite the various kinds of bad conscience.
594. Definition.—The distinction of good and bad conscience is applied both to consequent and antecedent conscience (see 576).
(a) The consequent conscience is good, and one is said to have a good conscience, if it testifies that past acts were rightly performed, that past sins were forgiven, that one is in the friendship of God, etc.; “The end of the commandment is charity from a good conscience” (I Tim., i. 5); “War a good warfare, having faith and a good conscience” (ibid., 19). The consequent conscience is bad if it testifies in a contrary way: “Let us draw near with a true heart, having our hearts sprinkled from an evil conscience” (Heb., x. 22).
(b) The antecedent conscience, with which we are now concerned, judges about the morality of an act to be performed here and now, or in the future. It is called good, if it is made by one who is in good faith—that is, one who sincerely loves goodness and who decides according to the truth as far as he is able to see it. It is called bad, if it is the judgment of one who is in bad faith—that is, one who is in error through his own fault, or who arrives at the truth by reasonings that are not honest or not understood by him. Example: Speaking of those who, though fearing that idol meats were forbidden, yet ate of them because they saw others do this, St. Paul says: “There is not knowledge in everyone. For some until this present, with conscience of the idol, eat as a thing sacrificed to an idol, and their conscience being weak is defiled” (I Cor., viii. 7).
595. Divisions.—By training and care a good conscience is developed and becomes better. (a) A vigilant conscience is one that asserts itself promptly and strongly under all circumstances. (b) A tender conscience is one that inclines to a careful observance of all the Commandments and to a purification of the inner workings of the soul. A possessor of this kind of conscience is called conscientious. (c) A timorous conscience moves one through filial fear to shun even the slightest sins and imperfections, and to use all prudent efforts to avoid occasions and dangers of sin. The possessor of this kind of conscience is called God-fearing.