From the time the apostles started writing to today, how did the New Testament come to be? It happened over four periods: 

• AD 30 – 100 (SCATTERED WRITINGS AND LIVING WITNESSES): 

Because witnesses of Christ were still alive there wasn’t a pressing need felt for a new canon of scripture. The people still had the Old Testament as well as the writings of the apostles in the four gospels and letters of Paul that circulated among believers. 

• AD 100 – 150 (WRITINGS SPREAD WIDELY AND ORAL HISTORIES): 

New Testament writings spread more widely but no one seemed to feel a need to canonize them. There were still living witnesses to the apostles of Jesus Christ and so many relied on the oral histories that these witnesses shared. 

• AD 150 – 350 (END OF LIVING WITNESSES, NEW TESTAMENT TAKING SHAPE): 

All living witnesses of Jesus and the apostles were now dead and so written records became more important than oral traditions. By 170 AD twenty of the twenty-seven New Testament books were already recognized as scripture, and around AD 200 the phrase “New Testament” was being used to refer to these scriptures. The book was now also being translated into other languages, helped by the formal acceptance of Christianity as a religion by the Roman empire. 

• AD 350 – 397 (FORMALLY CANONIZED NEW TESTAMENT): 

Realizing the need for agreement on such issues as the nature of Jesus Christ and God, Christian leaders began to hold councils. During these councils, the New Testament with its twenty-seven books was officially recognized as canonized scripture. (Source)