Neither living nor lifeless faith remains in a heretic who disbelieves one article of faith.The man who refuses to believe even one article has effectively demonstrated that he does not hold the Church's teaching on faith and morals to be infallible. But if he has denied this infallibility, then he has no better reason for what he believes other than his own will - what he chooses to believe or not, on his own authority. In this way he has ceased to be Catholic.
The reason of this 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 the teaching of the Church as to an infallible rule, but to his own will. Hence it is evident that a heretic who obstinately disbelieves one article of 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 the other articles, but only a kind of opinion in accordance with his own will (ST II-II Q5 A3).
We are not free to pick and choose that which we will believe from what the Church teaches. The Catholic Faith is not a smorgasbord. We are obliged to believe everything that the Church proposes for belief. It is truth. To deny this truth on any point is to say that the Church lies. But that is impossible, for her Head is the Lord Jesus Christ himself. The Church cannot err precisely because she is totus Christus (CCC 795) with him - Head and Body. It's not that she is infallible in and of herself - hardly! Rather, it is because she is united with her Head. To say that the Church could err would be the same thing as to say that Christ himself can err.