Objection 1. It would seem that apostasy does not pertain to unbelief. For that which is the origin of all sins, does not, seemingly, pertain to unbelief, since many sins there are without unbelief. Now apostasy seems to be the origin of every sin, for it is written (Sirach 10:14): "The beginning of the pride of man is apostasy [Douay: 'to fall off'] from God," and further on, (Sirach 10:15): "Pride is the beginning of all sin." Therefore apostasy does not pertain to unbelief.
Objection 2. Further, unbelief is an act of the understanding: whereas apostasy seems rather to consist in some outward deed or utterance, or even in some inward act of the will, for it is written (Proverbs 6:12-14): "A man that is an apostate, an unprofitable man walketh with a perverse mouth. He winketh with the eyes, presseth with the foot, speaketh with the finger. With a wicked heart he deviseth evil, and at all times he soweth discord." Moreover if anyone were to have himself circumcised, or to worship at the tomb of Mahomet, he would be deemed an apostate. Therefore apostasy does not pertain to unbelief.
Objection 3. Further, heresy, since it pertains to unbelief, is a determinate species of unbelief. If then, apostasy pertained to unbelief, it would follow that it is a determinate species of unbelief, which does not seem to agree with what has been said (10, 5). Therefore apostasy does not pertain to unbelief.
On the contrary, It is written (John 6:67): "Many of his disciples went back," i.e. apostatized, of whom Our Lord had said previously (John 6:65): "There are some of you that believe not." Therefore apostasy pertains to unbelief.
I answer that, Apostasy denotes a backsliding from God. This may happen in various ways according to the different kinds of union between man and God. For, in the first place, man is united to God by faith; secondly, by having his will duly submissive in obeying His commandments; thirdly, by certain special things pertaining to supererogation such as the religious life, the clerical state, or Holy Orders. Now if that which follows be removed, that which precedes, remains, but the converse does not hold. Accordingly a man may apostatize from God, by withdrawing from the religious life to which he was bound by profession, or from the Holy Order which he had received: and this is called "apostasy from religious life" or "Orders." A man may also apostatize from God, by rebelling in his mind against the Divine commandments: and though man may apostatize in both the above ways, he may still remain united to God by faith.
But if he give up the faith, then he seems to turn away from God altogether: and consequently, apostasy simply and absolutely is that whereby a man withdraws from the faith, and is called "apostasy of perfidy." On this way apostasy, simply so called, pertains to unbelief.
Reply to Objection 1. This objection refers to the second kind of apostasy, which denotes an act of the will in rebellion against God's commandments, an act that is to be found in every mortal sin.
Reply to Objection 2. It belongs to faith not only that the heart should believe, but also that external words and deeds should bear witness to the inward faith, for confession is an act of faith. On this way too, certain external words or deeds pertain to unbelief, in so far as they are signs of unbelief, even as a sign of health is said itself to be healthy. Now although the authority quoted may be understood as referring to every kind of apostate, yet it applies most truly to an apostate from the faith. For since faith is the first foundation of things to be hoped for, and since, without faith it is "impossible to please God"; when once faith is removed, man retains nothing that may be useful for the obtaining of eternal salvation, for which reason it is written (Proverbs 6:12): "A man that is an apostate, an unprofitable man": because faith is the life of the soul, according to Romans 1:17: "The just man liveth by faith." Therefore, just as when the life of the body is taken away, man's every member and part loses its due disposition, so when the life of justice, which is by faith, is done away, disorder appears in all his members. First, in his mouth, whereby chiefly his mind stands revealed; secondly, in his eyes; thirdly, in the instrument of movement; fourthly, in his will, which tends to evil. The result is that "he sows discord," endeavoring to sever others from the faith even as he severed himself.
Reply to Objection 3. The species of a quality or form are not diversified by the fact of its being the term "wherefrom" or "whereto" of movement: on the contrary, it is the movement that takes its species from the terms. Now apostasy regards unbelief as the term "whereto" of the movement of withdrawal from the faith; wherefore apostasy does not imply a special kind of unbelief, but an aggravating circumstance thereof, according to 2 Peter 2:21: "It had been better for them not to know the truth [Vulgate: 'the way of justice'], than after they had known it, to turn back."
Source: Summa Theologica (New Advent)