Chapter VI: Voluntary and Involuntary Sins, § 20.
CHAPTER VI: ON VOLUNTARY AND INVOLUNTARY SINS
§ 20. The first division was this: some sins are involuntary, namely, those which are committed out of ignorance or weakness; some are voluntary and are committed out of the deliberate malice and stubbornness of the mind. Augustine (83 Quaest., q. 26): “Some sins are sins of infirmity; others, of inexperience; others, of malice.” The basis for this division is found in the following passages of Scripture: Num. 15:27–28 and 30–31: “If a soul sins through imprudence, he shall offer a female goat a year old for a sin offering. And the priest shall make reconciliation before the Lord for the soul who sins through imprudence, that is, because of ignorance. But the soul who does anything with a high hand blasphemes the Lord; he shall perish from his people because he has despised the Word of Jehovah and has made His precept of no effect.” Ps. 19:12–13: “Who discerns his errors? Cleanse me from hidden [faults], and keep me from acts of pride.”
Involuntary, unwilling sins are so called when a person sins out of the weakness of the flesh through inadvertence or out of ignorance. In Num. 15:24–25, 27–29 they are called 
[“mistake”], while the Septuagint calls them ἁμαρτία ἀκoύσιoς [“unintentional sin”]. In Ps. 19:12 they are called 
[“errors”]; in the Septuagint, παραπτώματα [“mistakes, trespasses”].
Those are called “voluntary” which someone commits knowingly and intentionally. In Num. 15:30 they are said to be 
, ἐν χειρὶ ὑπερηφανίας [“acts done with a high hand”], and in Ps. 19:13, 
, “sins of pride.”
§ 21. Examples of sins committed out of ignorance and unintentionally occur in many places throughout the Scriptures. In Gen. 20:2 King Abimelech of Gerar takes Sarah, and in v. 6 God says to him in a dream: “I know that you have done this in the integrity of your heart, and therefore I have kept you from sinning against Me.” Lev. 4:2: “The soul which has sinned by mistake” (through ignorance or inadvertence). In Num. 35:15 “cities of refuge” are established for those “who have killed by error.” Acts 3:17: “I know, brothers, that you have acted through ignorance.” 1 Tim. 1:13: “I have received mercy because I acted ignorantly.” Many passages of Scripture speak about this kind of sin. 1 Kings 8:46; 2 Chron. 6:36; Eccl. 7:[20]: “There is no person who does not sin.” Prov. 20:9: “Who can say, ‘I am clean of sin’?” Gal. 6:1: “If someone has been overtaken in any trespass, in some error, he should be restored.” James 3:2: “We all err much.” Augustine (De peccat. merit. et remiss., ch. 17): “Ignorance and weakness are vices because they hinder the will from being stirred to do a good work or from abstaining from an evil work.”
§ 22. Concerning sins committed willingly out of intended malice, by choice and stubbornness (which otherwise are said to be sins against conscience), the following passages speak: Rom. 6:15: “Shall we go on to sin because we are not under the Law? By no means!” Titus 3:10–11: “Avoid a heretic after one or two admonishments, knowing that such a person is overturned and sins as one self-condemned.” 2 Pet. 3:5: “They deliberately ignore that the heavens existed beforehand,” etc. Here some assign the words of Heb. 6:4ff. and Heb. 10:26, but in its own place [§ 108] we shall show that the discussion in those passages refers to the sin against the Holy Spirit, which is unforgivable, though the rest of the voluntary sins are forgivable and are forgiven if one turns away from them through repentance (Lev. 6:1ff.).
Twofold ignorance.
§ 23. Furthermore, ignorance is twofold. One sort is [ignorance] of the act, but another is [ignorance] of the Law; one, invincible, but one, studied. The former excuses and lessens the seriousness of the sin in a way, but not the latter. Luke 12:48: “The servant who did not know the will of his master and did what deserved a beating will be beaten with few strokes.” Augustine (De grat. et lib. arb., ch. 3):
It is more serious for a person to sin knowingly than in ignorance. Nevertheless there is no refuge to be sought in the shadows of ignorance, as though someone could find an excuse in them. Not knowing is one thing, but being unwilling to know is another. After all, the will is accused in the case of a person of whom it is said, “He is unwilling to know how to act well.” But even the ignorance which belongs not to those who are unwilling to know but to those who, so to speak, simply do not know does not excuse anyone in such a way that he would not burn in eternal fire.
Again (QQ. V. ac N. T., q. 67): “Not every ignorant person is immune from punishment, but that ignorant person can be excused from punishment who has not found someone from whom to learn.” Retract. (bk. 1, ch. 13):
These sins are not undeservedly called voluntary sins, because they are committed by people who are either ignorant or have been compelled, but they cannot be committed without any will whatsoever, since he who sins ignorantly certainly sins voluntarily, because he thinks that something which must not be done should be done. But the person who does not do the things he wants to do because the flesh lusts against the Spirit indeed lusts unwillingly, and in that point he does not do what he wishes. But if he is overcome, willingly assenting to his concupiscence and in this does nothing
(except)
what he wills, he is obviously without righteousness and a slave of sin.
This is why Thomas ([ST,] 1.2., q. 76, art. 2) distinguishes between “ignorance [ignorantia] and not knowing [nescientia].” Not knowing indicates a simple negation of knowledge, but ignorance introduces the privation of knowledge, namely, when someone lacks a knowledge of those things he was born apt to know.
A sin of malice is more serious than a sin of ignorance or weakness.
§ 24. Surely a sin committed out of deliberate malice is far more serious than sins of weakness; Thomas ([ST,] 1.2., q. 78, art. 4) proves this in three ways:
First, sin exists principally in the will; therefore the more the movement of the sin is proper to the will, the more serious is the sin, all other things being equal. In a sin committed out of malice, the will is moved of itself, but passion is an extrinsic impulse. Second, passion which inflames the will passes quickly, but the habit because of which a person sins out of malice is a quality which remains. Third, whoever sins out of malice is ill-disposed to the very end, which is the principle1 in matters of action, and thus his failing is the more dangerous.
1 See above, p. 14 n. 7.