There is that which MUST be believed, for it is revealed in the Sacred Scriptures as divinely true.
There is that which MUST NOT be believed, for it contradicts what is revealed in the Sacred Scriptures as divinely true.
There is that which MAY be believed, for it does NOT contradict what the Sacred Scriptures reveal as divinely true, but in fact harmonizes with that revelation, and has a long history of fellow Christians believing it across the centuries.
It strikes me that the third thing, the third point, is what St. Thomas Aquinas called "probable" but not "incontrovertible" as he said in the Summa:
Nevertheless, sacred doctrine makes use of these authorities as extrinsic and probable arguments; but properly uses the authority of the canonical Scriptures as an incontrovertible proof, and the authority of the doctors of the Church as one that may properly be used, yet merely as probable. For our faith rests upon the revelation made to the apostles and prophets who wrote the canonical books, and not on the revelations (if any such there are) made to other doctors. Hence Augustine says (Epis. ad Hieron. xix, 1): "Only those books of Scripture which are called canonical have I learned to hold in such honor as to believe their authors have not erred in any way in writing them. But other authors I so read as not to deem everything in their works to be true, merely on account of their having so thought and written, whatever may have been their holiness and learning."--Summa Theologia, Part 1, Question 1, Article 8
I believe that the Orthodox use the term "theologoumena" for something akin to the third category, and by that term mean something stronger than merely what Lutherans mean when they tend to dismiss something as "just pious opinion." Thoughts on Pr. Webber's categories?
[I note Gerhard using the very category most reverently in his homily for Quasimodo on the question of whether the scars of our Lord remain visible in His holy body to this day. "Yet with this it should be noted that this actually should not be regarded as an article of faith, because there exists no express, clear testimony from Scripture about this. Rather, it is only a presumptive conclusion." And yet he goes on to speak of the matter with great reverence for the way the ancient teachers laid down their reasons for so concluding and clearly agrees with them and includes them in his sermon - Postilla I:364-366]