Most of the books of the New Testament that we use today were written by around 100 AD. However, there were also many other documents around at the time that didn’t make it into the Bible. There were many groups in the early church that had heretical beliefs. For example, the Gnostics believed that the Bible contained “hidden wisdom” that could only be uncovered through intensive study. ANYONE who could uncover these secrets would be admitted into heaven, whether they were Mother Teresa or Adolf Hitler. They also believed that anything made of substance (the fleshly world) was evil and corrupt, and that only the spiritual (the realm of thought and the mind) was good. Therefore, they rejected that God could possibly have become man, because he would never have denegrated himself to a form made out of flawed, evil flesh. The Gnostics wrote many scriptures, like the Gospel of Thomas, later in order to validate their ideas. Other books were written out of a desire for ordinary people to learn more about the missing parts of Jesus’s life. These folk tales were clearly invented, but many were written down, like the Infancy Gospel of Thomas, which tells about what Jesus did from his birth to the age of 12. Still other books discussed the early church in a faithful manner and were used to teach, but were simply not inspired. Books in this category include the Shepherd of Hermas, the Epistle of Barnabas, and the Didache.
So, for three hundred years, traditions were passed along orally through the successors of the Apostles, but debate continued about WHICH Scriptures were inspired. Although there was general consensus about the four Gospels and most of Paul’s letters, some locations in the Roman Empire were including some of these false scriptures, and others were rejecting ones that we use today (like 2 Peter and Revelations). The vast distances in the Roman Empire made it difficult for the isolated and persecuted underground Christian communities to keep things standardized, which made the appointment of Bishops, which represented their congregations who traveled frequently to discuss doctrinal matters with other communities, very important.
As a result of all of this confusion, by the early 300s, another heresy was growing due to the inability to decide upon scripture. Some Christians were rejecting the Trinity, believing that Jesus was only a man. They were called Arius. So, a council was called by the Emperor Constantine to hash out Christian doctrine once and for all. This was the famous Council of Nicea in 325, which gave us the Nicene Creed which we use to define Christians today.
The Catholic (catholic means “universal”, and refers to the fact that there was ONLY one, unified denomination at first) bishops also realized that it was time to definitively figure out what was scripture and what wasn’t, and so a series of other councils were held in the late 300s, including the Councils of Carthage, Hippo, and Rome. This finally standardized the Bible by around 400 AD. BUT… there was STILL a problem! The Old Testament was written in Hebrew, while the New Testament was written in Greek. Only if someone could read BOTH languages (which was very uncommon) could a person actually read ALL of the scriptures for themselves. Only when St. Jerome finally completed his Latin translation did a single version in a single language exist, in the early 400s AD.
So, for 400 years, it was Catholic bishops that held the church together and decided doctrine. It was Catholic bishops that determined what books were inspired. They did this because the Catholic Church had the protection of the Holy Spirit, to ensure that their choices were accurate. We believe that the bishops continue to hold that power today, and that Sacred Tradition is as important as the Bible (which is why some traditions that we believe are not explicitly in the Bible).
So, if it wasn’t for the Catholic Church, there wouldn’t even BE a Bible!