Why does everything Christians believe have to be in the Bible?

Protestants and Evangelicals often accuse Catholics of "not following the Bible". To which I ask, where is it clearly and explicitly stated - or even just implied - that everything Christians believe, say, and do must be clearly and explicitly stated in the Bible? So far as I can tell, it's not.

It's for this reason, among others, that the Church, in addition to the Bible, also appeals to tradition and its teaching authority.

Why does everything Christians believe have to be in the Bible?
Why does everything Christians believe have to be in the Bible?
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