41 If in the difficult times in which Our lot is cast, Catholics will give ear to Us, as it behoves them to do, they will readily see what are the duties of each one in matters of opinion as well as action. As regards opinion, whatever the Roman Pontiffs have hitherto taught, or shall hereafter teach, must be held with a firm grasp of mind, and, so often as occasion requires, must be openly professed.
Source: Immortale Dei (Vatican.va)