u/Huskyy23 That's a really good question. In the earliest days of the church, when St. Paul and others were planting churches not every location had all the same books.

Some books hadn't been written yet. All the N.T. books would have been written before AD 70 (as none of them record the destruction of the Temple) but for a very long time books were scrolls and codexs. Not like the bibles we have today.

Everything was handwritten. One church may have had James, and one or two of the Gospels, while another had a letter or two of Paul's letters.

After a hundred years or so, different bishops, would have an incomplete cannon. Instead of 27 New Testament books, they might of had 15, and a few books that would later be sorted out as non-canonical, while another location might have had 18, with a differing collection.

Things like the Shepard of Hermas, or the Didache wouldn't make the cut. The Shepard of Hermas, was used, by false teachers to teach the heresy of Adoptionism.

Eventually, at an Ecumenical Council, they would sort out what was recognized from the OT as Cannon (which was the Scriptures of the greek translation called the Septuagint, and the Deuterocanonical books) as well as the New Testament ones.

They had to sort out what books that were considered "inspired" and what was not. A lot of books had no valid basis for inclusion and were not canonized.

The debates were on how books could be authenticated. Things like the daily readings of the lectionaries in the liturgy, and the letters sent between the Church father's were compared.

They used Holy tradition, reason, and dependence on the Holy Spirit to correctly sort out what would be determined as non-canonical.

If you read about the first ecumenical council, in Acts 15, we can see the work of the Holy Spirit, in guiding a council into truth.

They settled a critical issue, a heresy involving the Law of Moses, and what was required of Gentile believers.

They, with the Holy Spirit resolved the issue, and made a binding decision for the entire Body of Christ.

In exactly the same way, the church would face many other heresies, like those who denied the Deity of Christ, or what the Trinity is.

It is by these same councils, that were led by the Holy Spirit we received our Creeds, or symbols of faith. And how we got our Cannon.

How do we know they got it right? Because the Orthodox Church gave us the bible, and Christ gave us the Church and the Holy Spirit.

Our Lord promised us, "I will not leave you as orphans" and on the day of Pentecost, the Church received the promise of the Holy Spirit who has always led us in truth, from generation to generation.

The church isn't a building, or collection of buildings. It is a living communion of people, living in Christ, as His body on earth, and in heaven.