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.
Note that this isn't a matter of interpreting or opining upon Divine Revelation; it's simply manifest, from the meaning of the terms, that who these days is called a "Cafeteria Catholic" adheres to his own will and not to the teachings of the Church.
And before anyone points out that "Cafeteria Catholic" is a derogatory term, let me point out that there are not only people who brag about adhering to their own will and not to the teachings of the Church, there are people who have made careers out of bragging about adhering to their own will and not to the teachings of the Church.
That people who adhere to their own will and not to the teachings of the Church are heretics is not based on Divine Revelation, nor the opinion of celibate old men in dresses in the Vatican, but from the definition of "heretic." The opinion of celibate old men in dresses in the Vatican is merely that being a heretic is not good.
St. Thomas concludes his response with:
Therefore it is clear that [an obstinate] 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.
The evidence that this is true in particular cases is often quite obvious. I need hardly add that holding opinions in accordance with your own will is not what the Church regards as proper exercise of conscience.