Replies

John 20:30 Sufficient for Salvation. Bam!
John 20:30: "Now Jesus did many other signs in the presence of the disciples, which are not written in this book."

More like a desperate whimper than a "Bam!" As I mentioned above about the Book of Revelation, all of the books of the Bible were written individually. This includes John's Gospel, so this verse was intended to apply only to that book.

In addition to this, even if you add the next verse:

...but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in his name.

...this passage says nothing about the sufficiency of the Bible as a whole as a rule of faith. It doesn't even claim it for the Gospel of John, only that Jesus did other things that were not included in it. Also, that would mean we would only need the Gospel of John. I am sure you would agree that that would be ludicrous.