At the time when the New Testament was compiled there was already a long standing tradition about what was authentic and what wasn't.
many were accepted, but there was still a LOT of debate about some. For example, the Arian Christians used different texts which also had an equally long history and pedigree. The Nestorian Christians rejected the council of Nicea, used yet a different selection of books, and were an integral part of the preservation of Greek and Roman knowledge.
And, of course, this doesn't even address the Gnostics, who again had a different collection of texts, had very different beliefs about the humanity of Jesus, and whose texts were just as old and supported as those for the Orthodox Christians.
Each group of Christians considered the other groups to be heretical. They had their own councils, their own texts, their own bishops, etc.
it wasn't until the emperor Constantine put his support with those from the council of Nicea that the Orthodox became the dominant sect. But even after that, the Arians continued in many areas, even being persecuted by the Orthodox. For a while the area of modern Spain was Arian Christian.