Of course you know, what you are going to get

, but anyway I take your challenge. Not in the sense that I will convince you ( I know that it will not happen). I just try to explain this from a Lutheran standpoint:
Regarding the authority of the OT, we have Jesus’ own words (Matt 5, 17-19). We can also see int the Bible that the first Christians recognised the OT as authoritative word of God (1 Timothy, 13; 2 Timothy 15 - 17).
Regarding the NT, Luke specifically says that his motivation to write his Gospel was to convince Theophilos (and naturally all the readers) how reliable and accurate the (presumably) oral teaching they have received is (Luke 1, 1- 4). In other words the Scripture was put as a standard for the accuracy of the teaching.
The Scriptures do not contain everything of Jesus’ life (18 years are missing) and they do not record His every act and word even during His ministry. According to John, “the world would not contain” all the books that should be written, if everything were recorded (John21, 25). This being the case, of course the most essential and most important facts were recorded. It would have been absurd to leave out anything that was necessary to Salvation and use the precious time and papyri to write details of secondary importance, especially, when the intention was to write so that the reader " might believe that Jesus is Christ, Son of God, and that believing you may have life in His Name" (John 20, 31).
So, while we do not know everything about Jesus and apostles, we have an apostolic quarantee that all essential was written down. Moreover, when we look at the postapostolic wriitngs of Ignatios or Polycarp, there is no indication of any extrabibilical tradition regarding the doctrine or practice. All the references that are discernible in the text refer to Scriptures.
There was also a certain suspicion to some, presumably Jewish oral traditions circulating among the first Christians (1. Timothy 1:4,4:7, etc).
Rest my case.