Many think the Catholics determined what books were to be included in the Bible, because they over the centuries publicly listed the books that they used. There were canons put together and used by people even before the Catholics. Different people gave personal statements about the books, but they were only commenting on the books and letters that the first Christians used from the beginning. They had only acknowledged those books early Christian communities already accepted as scripture.
The Catholic denominations started to introduce heresies in approximately 310 A.D. The Roman Catholic Church, which taught things contrary to the Bible, began with the prayers for the dead and the sign of the Cross. The Catholics continued to bring in many false teachings.
Official canonization of the New Testament scriptures came about because of heresies Gnostics and other sects spread. The first Christians accepted as scripture New Testament teachings by letter and books right from the beginning.
The New Testament teachings were by letter and books right from the beginning. In 1 Timothy 5:18 Paul joins a New Testament scripture (Luke 10:7) to an Old Testament scripture (Deuteronomy 25:4) and calls them both scripture. In addition, we can see in 2 Peter 3:15-16 Peter recognizes what Paul writes as scripture.
Again, the believers from the beginning used these books and letters from the start. That is what determined these books as scripture.