Apparently, as far as I can find out, the Ekklesia in the first century, the Apostles, and later the Waldensians and the Anabaptists, and others, believed the Bible is the TRUTH, and would not believe things outside the Bible that contradict the Bible.
You found out wrong regarding the first Century; the Bible existed in bits and pieces all over Christendom and as we know it, was not even compiled until the 3rd century. Even then, the canon of Scripture remained "open" which explains the variation in content of Bibles in Western, Eastern, and Oriental Orthodox. The Waldensians used the Vulgate; the Anabaptists Luther's Bible; both of which have more books than what protestants would consider "The Bible".
The canon remained open until the Council of Trent (Catholic) and various reformed protestant sects closed it. The Orthodox, Oriental, Lutherans (Confessional Lutherans for sure; in Concordia's latest edition of the Apocrypha contains 21 books over and above the 66 in the protestant Bible
The Apocrypha: The Lutheran Edition with Notes), and some (most?) Anglican communities still retain an open canon.
Good Article here:
Books of the Bible - Wikipedia
Now, not all books are "equal"; Gospel supersedes the Law; The Gospels are the source of doctrine; Acts and the Epistles teach and support the Gospels; some theological concerns remain about certain epistles and the Revelation, due to questions of indefinite authorship.
So, before everyone starts spouting off, we Confessional Lutherans
are "Sola Scriptura" and we do hold Tradition only as far as those traditions are neither at odds with, nor are forbidden by, Scripture. If Scripture is silent regarding a long held traditional belief, then it is Adiaphora; a thing of indifference; we are free to accept or reject it. Adiaphora can be things like the Assumption of Mary; a set lectionairy, vestments, how many or any candles on the Altar, presence/absence of artwork in our Churches etc. The Eucharist is clearly supported and mandated in Scripture; and liturgical worship was and remains the norm of the true Church as it was mandated in the old testament, and continues according to God's will.
The 66 book "Protestant" Bible came about as a puritanical British response against the taxation of paper and printed material. By excluding the Apocrypha from the KJV, the cost per book was greatly reduced. (as we are fond of doing; we can blame indiscriminate and unfair taxation for the protestant Bibles of today; a very "Christian" reason for a closed canon eh?). So, the idea of a closed canon is a recent innovation, not driven by the Church, but by a secular law.