Odd that it teaches a doctrine about Scripture which is found nowhere in Scripture.
..and to which doctrine might you be referring?
Actually, Scripture does contain all we need for salvation.
John 20:30-31 30 Now Jesus did many other signs in the presence of the disciples, which are not written in this book; 31 but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.
Actually, all we need to know to believe is what's written in John's gospel.