"To reject any article of the faith is to reject the faith itself. This is like pulling one stone out of an arch; it is like putting one hole in the hull of a ship. The whole arch tumbles down; the whole ship sinks. A man who has the faith, accepts God's word. Now, God's word has set up the Church as man's infallible teacher and guide. If a man, therefore, rejects one article of the faith, and says that he believes all the other articles, he believes these by his own choice and opinion, not by faith. Rejecting one article of the faith, he rejects the whole authority of the Church, and he rejects the authority of God who has set up and authorized the Church to teach truth. Hence, it is entirely incorrect to say that am man may have lifeless or formless faith in some article of the Creed while he rejects others; such a man has not the faith at all, living or lifeless." Thomas Aquinas, Summa IIa IIae Q5
Sorry, I lost the source of the above quote, which is based on the Summa, as opposed to IN the Summa.