How was it decided what books to include in the New Testament?

Answer 1

Ireneus insisted that there must be four and only four gospels and that those were Matthew, Mark, Luke and John, to the exclusion of other gospels that had been written. Because Acts of the Apostles was written by the author of Luke and appeared to be the most comprehensive history of the early Church, that too was included. Revelation was included because it was signed by a person called John, and it was decided that this was the apostle John.

A great deal of debate surrounded which epistles were genuine, and the present list was included because it was decided that these most probably were really written by the apostles. Hebrewswas included because, at that stage, it was thought to have been written by Paul.