841. Real doubt in matters of faith is always unjustifiable in itself, for there is never any just reason for doubting God’s word; but it is not always a sin of heresy or of infidelity. There are two kinds of real doubt, viz., the involuntary and the voluntary. (a) Doubt is involuntary, when it is without or contrary to the inclination of the will, or when it proceeds from lack of knowledge (see 40-55 on the Impediments to Voluntariness). Example: Indeliberate doubts, and doubts that persist in spite of one, lack the inclination of the will, while doubts that proceed from invincible ignorance lack knowledge. (b) Doubt is voluntary, when it is according to inclination and with sufficient knowledge.
842. Involuntary doubt in matters of faith is neither heretical nor sinful, for an act is not sinful, unless it is willed (see 99).
(a) Indeliberate doubts arise in the mind before they are adverted to and without any responsibility of one’s own for their appearance. From what was said above on first motions of the soul (see 129), it is clear that such doubts are not sinful.
(b) Unwelcome doubts persist in the mind after they have been adverted to, and, since faith is obscure (see 752, 799), it is not possible to exclude all conscious doubts, or even to prevent them from occurring often or lasting a considerable time. From what was said above on temptation (see 253 sqq.), it is clear that, if the person troubled with unwished doubts makes prompt and sufficient resistance, he not only does not sin, but gains merit. But, if his resistance is not all it should be, and there is no danger of consent to the temptation, he sins venially.
(c) Ignorant doubts occur in persons who have not received sufficient religious instruction, through no fault of their own, and who therefore regard the doctrines of faith as matters of opinion, or at least look upon doubts as not sinful. From what was said above on invincible ignorance (see 30), it is clear that such persons do not sin by their doubts.
843. Voluntary doubt is entertained either in ignorance for which one is responsible, or in full knowledge; in the former case it is indirectly voluntary, in the latter, directly voluntary.
(a) The doubts of one who is responsible for them because he did not use the means to instruct himself in the faith, are a sin of willful ignorance proportionate to the negligence of which he was guilty; but, if he is willing on better knowledge to put aside his doubts and accept the teaching of the Church, he is not pertinacious, and hence not guilty of heresy or infidelity.
(b) The doubts of one who is responsible for them, and not uninstructed or ignorant in faith, are sometimes positive, sometimes negative. Neither of these kinds of doubt is equivalent to heresy or infidelity in every case.
844. Negative doubt is the state of mind in which one remains suspended between the truth contained in an article of faith and its opposite, without forming any positive judgment either of assent to or dissent from the article, or its certainty or uncertainty.
(a) If this suspension of decision results from a wrong motive of the will, which directs one not to give assent on the plea that the intellect, while not judging, offers such formidable difficulties that deception is possible, then it seems that the doubter is guilty of implicit heresy, or at least puts himself in the immediate danger of heresy.