You belive in Scripture, correct?
Then why don't you believe this?
John 21: (We'll be using the KJV today to keep things on even footing): "And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written. Amen."
The Bible Itself declares that it doesn't contain everything.
There is a difference between everything and everything essential.
Again, I'll reference Polycarp, student of Paul, who wrote a letter to the Philippians. In that letter he states that Paul "wrote a letter unto you, into the which if ye look diligently, ye shall be able to be builded up unto the faith given to you,"
Polycarp was a disciple of the Apostle Paul. He says that just one letter of Paul is sufficient to build up the faith. Imagine what all of the letters of the new testament make possible?
Why are you twisting the meaning of what John wrote? He was talking about the many, many things Jesus did while He was with them. John did not say there was a separate revelation outside God's Word.
The Bible Itself declares that it doesn't contain everything.
This is true, but it doesn't give license to add any new revelations such as we see develop regarding Mary, indulgences, etc.