Joh_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.
1Jn_5:13 These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
Well that covers the material and formal sufficiency...What was the next question???
In order to prove sola scriptura a Protestant must prove the different and much stronger claim that Scripture is so clear that no outside information or authority is needed in order to interpret it.
2Pe_1:20 Knowing this first, that no prophecy of the scripture is of any private interpretation.
God says his scripture is materially and formally sufficient and any outside interpretation is not acceptable...That's good enough for millions upon millions of Christians who read and believe God...But to put a cap on it,
Joh 5:39 Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.
That's where you learn of God...Not outside of the scriptures but inside...