Hypothetical question: If someone had only the Scriptures, and no other books or access to outside teaching, would it be possible for them to become a mature and godly Christian?
According to the Bible outside teaching is necessary.
Philip ran up and heard him reading Isaiah the prophet, and said, “Do you understand what you are reading?” And he said, “Well, how could I, unless someone guides me?” And he invited Philip to come up and sit with him. Acts 8:30-31.
“Whoever will call on the name of the Lord will be saved.”
How then will they call on Him in whom they have not believed? How will they believe in Him whom they have not heard? And how will they hear without a preacher? Romans 10:13-14.