Do you allow the Mormon their beliefs as valid? The Muslim?
They also base their beliefs on writings not found in Scripture.
Roman Catholicism tells everyone their "Sacred Tradition" does not contradict Scripture and that their beliefs can be found in Scripture.
However, that is not the case. Take the Immaculate Conception as an example.
The Catholic Encyclopedia Online, which advertises itself as: "the most comprehensive resource on Catholic teaching, history, and information ever gathered in all of human history." has this to say about the IC.
No direct or categorical and stringent proof of the dogma can be brought forward from Scripture.
One of the few times the pope has spoken ex cathedra cannot be found in Scripture.
Staying with this topic...to further illustrate why Scripture is superior to "Tradition" the Catholic Encyclopedia offers examples of the contradictory positions of the various ECFs regarding the IC. They call these error. If you are aware of the ECFs these are not light weights in Roman Catholicism.
From the CE: In regard to the sinlessness of Mary the older Fathers are very cautious: some of them even seem to have been in error on this matter.
Origen, although he ascribed to Mary high spiritual prerogatives, thought that, at the time of Christ's passion, the sword of disbelief pierced Mary's soul ; that she was struck by the poniard of doubt ; and that for her sins also Christ died ( Origen, "In Luc. hom. xvii").
In the same manner St. Basil writes in the fourth century: he sees in the sword, of which Simeon speaks, the doubt which pierced Mary's soul (Epistle 259).
St. Chrysostom accuses her of ambition, and of putting herself forward unduly when she sought to speak to Jesus at Capharnaum ( Matthew 12:46 ; Chrysostom, Hom. xliv; cf. also "In Matt.", hom. 4).
The CE calls these "stray private opinions".
I don't care to debate the Immaculate Conception in this thread.
I merely use it as an example of a "Tradition" Rome claims to be true that is not found in Scripture and is contradicted among the very ECFs Rome leans upon.
https://www.catholic.org/encyclopedia/view.php?id=6056
Where in the bible did Christ say "go and write a book, call it the bible, and base your faith only on what is written in it?
Perhaps an interesting exercise would be to do a word study on "it is written" and "write" to see how many times and in what context those words are used.
The Psalmist wrote these words:
11Your word I have treasured in my heart, That I may not sin against You. Psalm 119:11 NASB
Kind of odd that I didn’t get a reply regarding the nonbiblical origin of sola scriptura. Hmmm..