We proceed to the third article thus:
1. It seems that a heretic who disbelieves one article of faith can have unformed faith in the other articles. For the natural intellect of a heretic is no better than that of a catholic, and the intellect of a catholic needs the help of the gift of faith in order to believe in any of the articles. It seems, then, that neither can heretics believe in any articles of faith, unless through the gift of unformed faith.
2. Again, the faith contains many articles, just as a single science, such as geometry, contains many conclusions. Any man can have a scientific knowledge of geometry in respect of some geometrical conclusions, even though he is ignorant of others. Similarly, any man can have faith in some of the articles of faith, even though he does not believe the others.
3. Again, just as a man obeys God in believing the articles of faith, so does he obey him in keeping the commandments of the law. He may be obedient in regard to some of the commandments, and not in regard to others. He may therefore have faith in regard to some of the articles, and not in regard to others.
On the other hand: as mortal sin is contrary to charity, so is disbelief in one article contrary to faith. Now charity does not remain after a single mortal sin. Neither then does faith remain after disbelief in a single article.
I answer: neither formed faith nor unformed faith remains in a heretic who disbelieves one article. The reason for this is that the species of any habit depends on what we mean by its formal object, and cannot remain if this is taken away. Now the formal object of faith is the first truth, as manifested in the sacred Scriptures and the doctrine of the Church, which proceeds from the first truth. Hence anyone who does not adhere, as to an infallible and divine rule, to the doctrine of the Church, which proceeds from the first truth manifested in the sacred Scriptures, does not possess the habit of faith, even if he maintains the things of faith otherwise than by faith. It is similarly obvious that one who maintains a conclusion without knowing the premise by means of which it is demonstrated has no scientific knowledge of it, but only an opinion. It is plain, on the other hand, that one who adheres to the doctrine of the Church as an infallible rule assents to everything that the Church teaches. For if he were to maintain such doctrines of the Church as he might choose, and not such as he did not wish to maintain, he would not adhere to the doctrine of the Church as an infallible rule, but only in accordance with his own will. It is clear then, that a heretic who persists in disbelieving one article of faith is not prepared to follow the doctrine of the Church in all matters. If he did not so persist, he would not be a heretic, but merely one who erred. It is thus apparent that one who is a heretic in one article does not have faith in the other articles, but holds an opinion in accordance with his own will.
On the first point: a heretic does not maintain the other articles of faith, in which he does not err, as a faithful man maintains them, that is, through adherence to the first truth, to which a man is bound to adhere if the habit of faith is in him. He maintains the things of faith according to his own will and judgment.
On the second point: the different conclusions of a science are proved by means of different premises, one of which may be known apart from the others. A man may therefore have scientific knowledge of some conclusions of a science without knowing others. Faith, however, accepts all the articles of faith on the ground of a single premise, namely, the first truth proposed to us in the Scriptures, according to the doctrine of the Church, which understands it properly. He who rejects this premise is therefore altogether without faith.
On the third point: the several commandments of the law may be considered in reference to their several proximate motives, in respect of which one of them may be kept and not another. But they may also be considered in reference to their single primary motive, which is perfect obedience to God, in which anyone fails who transgresses any one commandment, according to James 2:10: "whosoever shall keep the whole law, and yet offend in one point, he is guilty of all."