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.
2 Timothy 3:14-17 14 But as for you, continue in what you have learned and have firmly believed, knowing from whom you learned it 15 and how from childhood you have been acquainted with the sacred writings, which are able to make you wise for salvation through faith in Christ Jesus. 16 All Scripture is breathed out by God and profitable for teaching, for reproof, for correction, and for training in righteousness, 17 that the man of God may be complete, equipped for every good work.
Scripture contains all that is necessary to bring us to salvation. Jesus said so, John did, and Paul did.
That it's the final authority is a given as it is Truth and as such is reliable to be used as an absolute standard by which to measure any other claims.
It doesn't even have to be stated so explicitly to be recognized. Even in science, there are absolute standards by which other things are measured.
Amen!