(a) To be sufficient, diligence need not be as a rule supreme (i.e., it is not necessary that one employ every possible means to acquire instruction), for even the most conscientious persons feel they have used sufficient diligence when they have employed the usual means for obtaining religious instruction;
(b) To be sufficient, diligence should equal that which is employed by good people in similar circumstances. Thus, the unlearned who consult the learned or frequent the instructions provided for them, the learned who devote themselves to study as ordered and who seek assistance in doubts, are sufficiently diligent.
910. One who has not used sufficient diligence is sometimes responsible, sometimes not responsible.
(a) A person is not responsible for his ignorance and lack of sufficient diligence, if he conscientiously desires to have the proper amount of instruction, and has not even a suspicion that his studies and knowledge are not sufficient. Example: Titus, having gone through a very small catechism, thought that he understood Christian doctrine sufficiently and had done all that was required. But some years later he discovered, when examined, that he was ignorant of many important matters, and had entirely misunderstood others.
(b) A person is responsible for his lack of diligence and knowledge, if at heart he does not care to know, even though no fears or doubts about his ignorance disturb him. Examples: Balbus always felt religion a bore. At Sunday school he was daydreaming; now during sermons on Sunday he falls asleep. The result is that he has many infidel ideas, but doesn’t know it, and is not much concerned. Caius secured for himself an office, for which he is unfitted on account of his ignorance. But he enjoys his position so much, and cares so little about its responsibilities, that he does not even dream of his incompetence, and would not try to change things if he did.
911. Similar to negligence about the truths of faith itself is negligence about truths connected with faith. (a) An unbeliever is guilty of negligence when against conscience he fails to pray for light and to inquire or inform himself about the credentials of religion, revelation, and the Church; (b) a believer is guilty of negligence if he fails to seek answers to objections against faith, when thrown much in the company of objectors.
912. Like to sins of ignorance are the two sins opposed to the Gift of Understanding. (a) Dullness of understanding is a weakness of mind as regards spiritual things which makes it very difficult for one to consider or understand them. It is sinful inasmuch as it arises from over-affection for carnal things, especially the delights of eating and drinking. (b) Blindness of mind is a complete lack of knowledge of divine things due to the fact that one refuses to consider them lest one feel obligated to do good, or to the fact that one is so wedded to passion that one gives it all one’s attention (Ps. xxv. 4). Blindness is sometimes a punishment (Is., vi. 10; Wis., ii. 21); it is a sin when it is voluntary—that is, when carnal delights, especially lust, make one disgusted or negligent as to the things of faith. Abstinence and chastity are two means that greatly aid spiritual understanding, as is seen in the example of Daniel and his companions (Dan. i. 17).
Art. 3: THE COMMANDMENTS OF FAITH
(_Summa Theologica_, II-II, q. 16.)
913. Unlike the commandments of justice, which are summed up in the Decalogue, the commandments of faith are not given in any one place of scripture; but they may be reduced to three: (a) one must acquire knowledge and understanding of one’s faith according to one’s state in life and duties; (b) one must believe internally the truths of faith; (c) one must profess externally one’s belief.