“t is important to recall that the Church’s infallibility is not limited to its primary object, which is the deposit of faith — the body of truths revealed by God (dogmas). Both Vatican Councils have made it clear that the charism of infallibility extends also to whatever teaching is necessary for ‘devoutly guarding and faithfully expounding’ this revealed deposit. …
“In dealing with this point, the Catechism of the Catholic Church [para. 2035 citing LG 25 and Mysterium Ecclesiae] add a further clarification in order to rebut those recent theologians who have claimed that in questions of morality there are so many ‘grey areas’, nuances, and exceptions to ‘rules’ that the Church’s infallibility does not extend to moral teachings…
“This explicit mention of ‘morals’ is not of course an innovation on the part of the Catechism, since the scope of the Church’s infallibility has always been taught (as is seen in the solemn definition of Vatican I) to be ‘doctrine of faith and morals’ (fides et mores). Moreover, the Latin term ‘mores’ includes more than just ‘morality’. For us English-speakers, ‘morals’ and ‘morality’ refer to ethical norms, most of which are part of the natural moral law as well as being handed down in Scripture and/or Tradition. We find them expounded, for instance, in Part III of the Catechism and in courses of ‘moral theology’. But in the context of the term fides et mores, fides means ‘faith’ in the narrow sense of speculative (purely intellectual) beliefs such as those we profess in the Creeds, while mores, in contrast, covers all practical, behavioral norms and customs that Catholics are to observe; and the Church can discern some of these customs or practices to be immutably necessary for integral Catholic living and so teach this fact infallibly. Examples of such infallibly enjoined practices that are not also part of the natural moral law would be the observance of Sunday instead of Saturday under the New Covenant, and the need for infant baptism.”