Sola Scriptura is, in my opinion, niave at best. One has to be naive to beieve that the NT contains everything Jesus taught.
Sola Sriptura, it seems to me, implies that the teachings of our Lord Jesus Christ which were not written down are somehow "less divine" or "uninspired." It would follow that only that which Jesus spoke as inspired was worth recording. I would venture to say that everything Jesus said and taught was equally divine and inspired.
Of course Jesus said and did things other than that which is recorded.
John, the Apostle, explicitly tells us so ...John 20:30 And many other signs truly did Jesus in the presence of His disciples, which are not written in this book.But do not neglect to read on through the end of John 20 ...
John 21:25 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.John 20:31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.This verse (which acknowledges that Jesus said and did other things) also clearly presents the point that what we need for life through Jesus Christ is written.
We have no need to speculate as to what other things Jesus might have said or done. Certainly, as the texts say ... He said and did other things.
But what we need for life is written.
What the Apostles knew of Jesus' teaching, that was not written, is what they passed on to their successors is not speculation, but Sacred/Holy Tradition of the Church.
There is no need for us to speculate about something that is well known, and has been since Day One (the Pentecost).
What John is saying is true, all of it -- that the Lord did and said more than is written and that John's Gospel was written so that people might (not will, or the only way they can) believe that Jesus is the Son of God (written in 97 AD).
The Sacred Tradition is, like the Bible, incorruptable and is not to be confused with human traditions.
The Nicene Creed was formulated on the basisc of the oral, unwritten, Sacred Tradition, not on the Christian Bible, which did not exist yet.
The Christian Bible was complied by accurately selecting the inspired against non-inspired, orthodox against heretic writings. Without knowing the unwirtten Truth, such selection would not have been possible.
The Gnostic Gospel of Thomas, for instance, was rejected as heresy based on the correct and unwritten knowledge of the faith, and not through Scriptures, which did not come out for another two centuries.