To Tom's words, I add what I wrote some months ago:
[Many] Catholics often experience difficulties and doubts about certain doctrines inasmuch as there is no apparent consensus in the Church about whether those doctrines have in fact been definitively and thus infallibly taught by the Church. In other words, granted that the Church can teach definitively and thus infallibly, and has clearly done so in certain instances, it is not easy to identify all and only the doctrines that the Church has in fact so taught. Indeed, attempts to do so are always of limited usefulness. This is a major reason why the development of doctrine, as Newman explained it, is so important. By such a process, and often in response to error, the deposit of faith ever more clearly manifests itself in particulars where formerly it was not so clear. So long as “difficulties” about doctrines not solemnly defined take the form of being unsure what the Magisterium would decide about them were it so to decide, such difficulties are perfectly compatible with the virtue of faith. For one still retains, implicitly, that docility of mind which keeps one ready to accept as from God whatever the Magisterium does end up proposing definitively, even if that turns out to be different from what one now tentatively believes. But once a given Catholic, citing “the primacy of conscience” or some other presumed imperative, takes upon herself the burden of deciding which, among doctrines that have been consistently taught and pertain to the deposit of faith, but have not been solemnly defined, are to be accounted binding and which are not, then at that moment she has substituted private judgment for catholic judgment. Thus she has ceased to be Catholic in all but name. She is spiritually Protestant even if nominally Catholic.
For 'private judgment' substitute 'opinion'.