A group of Christians under the holy spirit would determine the same writings are scripture. Scripture is as self evident as a child knowing its mother.
A pile of rocks lying on the ground, raised to life as believers, under the Holy Spirit’s guidance would arrive at pretty much the same conclusions as a crowd of serious Christians.
Is it? In 325 the early Church had to answer what was perhaps its greatest controversy ever: the question of the Divinity of Christ. 2000 years later, one would think that a very clear answer came from Scripture – of course Christ was divine, of course the Trinity was a reality, of course Jesus was the second person of the Trinity. But this was a major battle in the Church. A priest named Arius and many of Eastern bishops believed that Jesus was not equal with God, and they had numerous Scripture passages and logical arguments that seem to strongly support that notion. Jesus Himself said many things that seemed to make it clear that he was lesser than the Father: the Father knew things that he did not, He did the will of the Father, Jesus prayed to the Father (was he talking to Himself?), etc. Also, logic seems to fight against the Trinitarian view: the eternal God of the universe was born as a baby, grew in knowledge, worked, obeyed his parents, went to the bathroom, got hungry and tired and so forth?
Christian positions (like the Trinitarian view) were not self-evident. In other words, while a strong argument for the Divinity of Christ could be made from Scripture. One could not pretend that it was the only possible interpretation. Indeed, Arius was not a monster; he was a generally well-liked priest that was genuinely concerned that the Church not misconstrue Jesus and His work. (The Jehovah’s Witnesses say the same thing today – using Scripture). The question is simply one of “anchor passages” – passages by which other passages are to be interpreted. We would point to John 1 (“The Word was God”) as clear evidence of the Divinity of Christ, but others would say that that is a heavily spiritual passage, and, given the clear indications from other verses that seem to show that Jesus is lesser than God, perhaps John 1 should be understood as meaning that The Word – this man who was born as a person – was “one with God” in some way. It all depends on which verses are considered “anchors.” If John 1 is an “anchor passage” (and I have interpreted it correctly), then the “Jesus as lesser being” passages should be interpreted as consistent with the “anchored” view that Jesus is God (i.e., is equal with the Father). But the reverse is also true – perhaps the “Jesus as lesser being” passages are the “anchors.” This raises the question: who gets to authoritatively decide which passages are the “anchor passages,” and what those passages mean in the first place?
Many, many gospels, books and letters were written by many people over the first few centuries. And the various churches had varying copies and disagreements about which should be considered “inspired.” There was some general agreement as to some of the books and letters. However, some churches considered certain books and letters inspired that were later considered not to be, by the Church. And some churches rejected certain books and letters later considered to be inspired by the Church. Who gets to define which are the “essential” beliefs? How did we got the NT? Who decided which books should be in the Canon of Scripture ... and ... by what authority did they make that determination?