Sacred Tradition IS revelation from God.
To believe that everything God revealed was written down by men is, ironically, a man-made tradition. No where does the Holy Book declare this to be so.
This belief was “made up later” (around the 1500s, when the Protestant Reformation began).
It will be a good logical argument for you to use, hasantas, whenever you are in dialogue with a fundamentalist Christian. You can say, “You believe that everything you are to believe as Christians must be found in the Bible, yes? So can you show me where this belief is found in the Bible?”
You will receive lots of obfuscation and circulatory remarks, but you won’t find a single Bible verse to support this.
(Well, they may say, “All Scripture is inspired by God”–2 Timothy 3:16 but that certainly does NOT say that everything we ought to believe is found in the Bible. It means that everything that’s in the Bible we should believe. Totally different.)
But since you are on a Catholic forum, you should know that Catholics do not believe that everything God revealed was written down…because that’s not anywhere to be found in the Sacred Scripture.