Objection 1. It would seem that there are not several species of unbelief. For, since faith and unbelief are contrary to one another, they must be about the same thing. Now the formal object of faith is the First Truth, whence it derives its unity, although its matter contains many points of belief. Therefore the object of unbelief also is the First Truth; while the things which an unbeliever disbelieves are the matter of his unbelief. Now the specific difference depends not on material but on formal principles. Therefore there are not several species of unbelief, according to the various points which the unbeliever disbelieves.
Objection 2. Further, it is possible to stray from the truth of faith in an infinite number of ways. If therefore the various species of unbelief correspond to the number of various errors, it would seem to follow that there is an infinite number of species of unbelief, and consequently, that we ought not to make these species the object of our consideration.
Objection 3. Further, the same thing does not belong to different species. Now a man may be an unbeliever through erring about different points of truth. Therefore diversity of errors does not make a diversity of species of unbelief: and so there are not several species of unbelief.
On the contrary, Several species of vice are opposed to each virtue, because "good happens in one way, but evil in many ways," according to Dionysius (Div. Nom. iv) and the Philosopher (Ethic. ii, 6). Now faith is a virtue. Therefore several species of vice are opposed to it.
I answer that, As stated above (I-II, 55, 4; I-II, 64, 1), every virtue consists in following some rule of human knowledge or operation. Now conformity to a rule happens one way in one matter, whereas a breach of the rule happens in many ways, so that many vices are opposed to one virtue. The diversity of the vices that are opposed to each virtue may be considered in two ways, first, with regard to their different relations to the virtue: and in this way there are determinate species of vices contrary to a virtue: thus to a moral virtue one vice is opposed by exceeding the virtue, and another, by falling short of the virtue. Secondly, the diversity of vices opposed to one virtue may be considered in respect of the corruption of the various conditions required for that virtue. On this way an infinite number of vices are opposed to one virtue, e.g. temperance or fortitude, according to the infinite number of ways in which the various circumstances of a virtue may be corrupted, so that the rectitude of virtue is forsaken. For this reason the Pythagoreans held evil to be infinite.
Accordingly we must say that if unbelief be considered in comparison to faith, there are several species of unbelief, determinate in number. For, since the sin of unbelief consists in resisting the faith, this may happen in two ways: either the faith is resisted before it has been accepted, and such is the unbelief of pagans or heathens; or the Christian faith is resisted after it has been accepted, and this either in the figure, and such is the unbelief of the Jews, or in the very manifestation of truth, and such is the unbelief of heretics. Hence we may, in a general way, reckon these three as species of unbelief.
If, however, the species of unbelief be distinguished according to the various errors that occur in matters of faith, there are not determinate species of unbelief: for errors can be multiplied indefinitely, as Augustine observes (De Haeresibus).
Reply to Objection 1. The formal aspect of a sin can be considered in two ways. First, according to the intention of the sinner, in which case the thing to which the sinner turns is the formal object of his sin, and determines the various species of that sin. Secondly, it may be considered as an evil, and in this case the good which is forsaken is the formal object of the sin; which however does not derive its species from this point of view, in fact it is a privation. We must therefore reply that the object of unbelief is the First Truth considered as that which unbelief forsakes, but its formal aspect, considered as that to which unbelief turns, is the false opinion that it follows: and it is from this point of view that unbelief derives its various species. Hence, even as charity is one, because it adheres to the Sovereign Good, while there are various species of vice opposed to charity, which turn away from the Sovereign Good by turning to various temporal goods, and also in respect of various inordinate relations to God, so too, faith is one virtue through adhering to the one First Truth, yet there are many species of unbelief, because unbelievers follow many false opinions.
Reply to Objection 2. This argument considers the various species of unbelief according to various points in which errors occur.
Reply to Objection 3. Since faith is one because it believes in many things in relation to one, so may unbelief, although it errs in many things, be one in so far as all those things are related to one. Yet nothing hinders one man from erring in various species of unbelief, even as one man may be subject to various vices, and to various bodily diseases.
Source: Summa Theologica (New Advent)