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We Proceed thus to the Third Article: It would seem that a heretic who disbelieves one article of faith, can have a lifeless faith in the other articles.
Obj. 1. For the natural intellect of a heretic is not more able than that of a Catholic. Now a Catholic’s intellect needs the aid of the gift of faith in order to believe any article whatever of faith. Therefore it seems that heretics cannot believe any articles of faith without the gift of lifeless faith.
On the contrary, Just as mortal sin is contrary to charity, so is disbelief in one article of faith contrary to faith. Now charity does not remain in a man after one mortal sin. Therefore, neither does faith, after a man disbelieves one article.
I answer that, Neither living nor lifeless faith remains in a heretic who disbelieves one article of faith.
The reason is that the species of every habit depends on the formal aspect of the object, without which the species of the habit cannot remain. Now the formal object of faith is the First Truth, as manifested in Holy Writ and the teaching of the Church, which proceeds from the First Truth. Consequently whoever does not adhere, as to an infallible and Divine rule, to the teaching of the Church, which proceeds from the First Truth manifested in Holy Writ, has not the habit of faith, but holds that which is of faith otherwise than by faith. Even so, it is evident that a man whose mind holds a conclusion without knowing how it is proved, has not scientific knowledge, but merely an opinion about it. Now it is manifest that he who adheres to the teaching of the Church, as to an infallible rule, assents to whatever the Church teaches; otherwise, if, of the things taught by the Church, he holds what he chooses to hold and rejects what he chooses to reject, he no longer adheres to an infallible rule, but to his own will. Hence it is evident that a heretic who obstinately disbelieves one article of the faith, is not prepared to follow the teaching of the Church in all things; but if he is not obstinate, he is no longer in heresy but only in error. Therefore it is clear that such a heretic with regard to one article has no faith in other articles, but only a kind of opinion in accordance with his own will.
Reply Obj. 1. A heretic does not hold the other articles of faith, about which he does not err, in the same way as one of the faithful does, namely by adhering simply to the Divine Truth, because in order to do so, a man needs the help of the habit of faith; but he holds the things that are of faith, by his own will and judgement.
-Summa Theologica, Part II-II, Q. 5, Art. 3

Whether Unbelief Is the Greatest of Sins?
We proceed thus to the Third Article: It would seem that unbelief is not the greatest of sins ...
I answer that, Every sin consists formally in aversion from God (Part I-II, Q. 71, Art. 6; Q. 73, Art. 3). Hence the more a sin severs man from God, the graver it is. Now man is more than ever separated from God by unbelief, because he has not even true knowledge of God: and by false knowledge of God, man does not approach Him, but is severed from Him. Nor is it possible for one who has a false opinion of God, to know Him in any way at all, because the object of the opinion is not God. Therefore it is clear that the sin of unbelief is greater than any sin that occurs in the perversion of morals as we shall state further on (Q. 20, Art. 3; Q. 34, Art. 2, ad 2; Q. 39, Art. 2; ad 3) ...
-Summa Theologica, Part II-II, Q. 10, Art. 3