The church, relying on apostolic tradition and the voice of the bishops, determined which books would be the New Testament canon. It was not an arbitrary decision that happened suddenly. But at the same time, the earliest Christians did not have a complete list, either. Most people acknowledge the NT canon being decided by the fourth century, with a few local church councils testifying to the current canon Christians use. In fact, in Catholic apologetics, we usually are glad to admit the slow and complex process, because it testifies to the authority of the church and (apostolic) Tradition.
While the church was able to decide which books were authentic, how did they decide that these books are in fact the inspired Word of God? How did they decide these books – the four gospels, acts, all the letters, and Revelation – were actually inerrant and God-breathed? It is one thing for these writings to be especially esteemed and held authoritative, being written by Apostles or their followers, and quite another thing to say these writings ultimately come from God.
Imagine, for example, that Christianity of today still didn’t have an official canon of the Bible. Pretend the church of today was much like the earliest church, which had apostolic writings that were revered and read in liturgy, but were not collected, compiled, and uniformly understood as an inerrant book of scripture. Now pretend the church of today wanted to officially decide which book were in fact * inspired* by God. Not just authoritative or inspiring but actually inerrant texts written by the Holy Spirit. How would the church decide that these special books are in fact inspired and not just special? Yes, a Catholic can say “by the church,” for Christ gave the church authority. But how would the question of inspiration come up?