The earliest New Testament consisted of the letters of Paul and the book of Luke. It is believed to have been compiled by a fellow named Marcion around 140 AD.
Marcion was considered a heretic because he did not believe the Old Testament spoke of the true God that Jesus revealed. He accepted only one gospel, the gospel of Luke, relying on Paul’s statement of there being one true gospel in Galations 1:6-7.
Many Bible scholars believe that most Christians believed Jesus would be coming back during their lifetime and therefore no “books” were written for future generations.
Logically speaking, there is no other way to understand it. I don’t know about you, but I’m not wasting any time on someone who promised a group of people over 2000 years ago that he would be back before their generation passed away. No one would believe it if they were not taught that is truth from an early age.
The teachings of Jesus should be more than enough for us to talk about, contemplate and emulate, without worrying about whether or not he is coming back, which does not make any sense anyway. There is no true death. We are spiritual. He is not gone.
Just because decades after the supposed life of Jesus, some unknown writers attributed some quotations to him, does not mean that Jesus truly said them.
The letters of Paul, known as the Pauline Epistles, were written by Paul to groups of worshipers he converted as he traveled the land spreading the gospel. He is widely considered to be the first missionary.