Coming close to heresy if the teaching is not scriptural. Here, Paul indicates the church is fallible:
But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. As we said before, so say I now again, if any man preach any other gospel unto you than that ye have received, let him be accursed (Gal 1:8-9).
And Jesus Himself, not qualifying "any man":
For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book (Rev 22:18-19).
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.