A-G: Because the indwelling Spirit brings the Scriptures alive within me as my eyes pass over the words.
That does not explain why you think the writings of the Apostles are *authoritative*; it only says that they "come alive" when you read them. The missing premise is that whatever comes alive to oneself as one is reading it should be treated as authoritative over oneself.
In other words, if you had been reading an article in Reader's Digest, and the words 'came alive' to you, would you then treat Reader's Digest as authoritative right along with the NT? It is an amazing coincidence that the books that have 'come alive' to you are (so far at least, I assume) only ones the Church teaches are authoritative. But if this experience (of having the words 'come alive') happens while you are reading something else (e.g. The Gospel of Thomas, Chicken Soup for the Soul, etc.), it seems that, to be consistent, you will have to add it to your personal canon. If someone else comes along and says that while reading the Gospel of Thomas the words "came alive" to him, you would have no way of showing him that he should not add the Gospel of Thomas to his own personal canon.
-A8
Reader's Digest . . .
CERTAINLY Holy Spirit is free to ENLIVEN, highlight, anoint . . . anything.
He actually anointed a donkey to speak sentences once upon a time. He MAY even have anointed a phrase or 3 from a Bishop or two sometime in history . . .
Certainly He can enliven something from The Reader's Digest, The Daily Times . . . something a snotty nosed kid blurted out on the playground as one passed by on the sidewalk.
Doesn't really matter the source. What matters is that Holy Spirit says at a given time and place to the individual concerned--
"You know that issue you've been so burdened about . . . remember that phrase the kid just blurted out on the playground? Apply that there. That's your answer."
And it is. And the burden lifts.
Was the kid enScripturating something new to be included in the Canon? Of course not. That's silly.
Was the highlight, anointed sentence Holy Spirit called the individual's attention to God's word to the individual at that moment? Absolutely.
With according fruitfulness in the individual's life.