Where does Scripture say it contains everything necessary for salvation?
The Christian faith existed and grew years before the first book of the New Testament was written.
The New Testament was written decades after Christ ascended into heaven. It took centuries for the Church to establish the Cannon of the New Testament. The early Church existed without a Bible.
Christ never told anyone to write a Bible. The Apostles never told anyone that Christianity would be based on a book.
And that from a child you have known the holy scriptures, which are able to make you wise unto salvation through faith which is in Christ Jesus. All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness (2 Tim 3:15-16).
Knowing this first, that no prophecy of the scripture is of any private interpretation. For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost (2 Peter 1:20-21).
But the early church didn't exist without Scripture. That was all available before someone had the idea to compile it into one handy book.
1 Corinthians 4:6 I have applied all these things to myself and Apollos for your benefit, brothers, that you may learn by us not to go beyond what is written, that none of you may be puffed up in favor of one against another.