This was a big issue for me as well in the beginning. Ultimately, it came down to pure logic and apostolic authority. I asked myself the question, “What came first? The Bible or the Church?” Once we answer this question, we can answer the source of Scriptural authority.
Christ gave authority to men (not Scripture) - “As the Father has sent me, so I send you.” How did the Father send the Son? - With ALL authority in Heaven and earth. Suffice it to say, men were given this authority and it is by this same authority that men were able to declare the Canon of Scripture.
Authority can only come from an equal or greater authority. So if all the Scriptures were not even written by the time of Christ’s ascension, then where did this authority come from? It can only have some from those who were authorized to grant it - otherwise Scriptural authority is a moot point.
God Bless,
CSJ