What part of the New Testament was written first?

The apostle Paul, purported to be the first author in the New Testament, had numerous letters or epistles throughout the Bible. The New Testament, the basis for Christianity, brings together numerous documents that recount early founders and beliefs of the Christian religion.
Many believe that the New Testament was written 100-300 years after Christ was crucified, however, it was written before the end of the first century by followers who either knew Jesus personally or those who had personal encounters with Him.
The Acts of the Apostles, describes the missionary endeavors of Peter and Paul is believed to be one of the first writings of the New Testament and was written by Luke.
Matthew, Mark, Luke and John, the first books of the New Testament, were actually written decades after the first letters written by Paul (Harris, 2014).

The epistles were the first pieces of the New Testament to be written. Chronologically, Paul’s epistles were written beginning in the mid-40s CE and continued for several decades. The Gospels were written around 70 CE and throughout that decade, while the epistles continued as the disciples aged, and John wrote his book of Revelation around 90 CE.
After Christ’s death and resurrection in roughly 30 CE, the gap of 15 years accounts for the spread of the Christian church, Paul’s conversion and commissioning as an evangelist. He spent several years studying and working with the disciples in Jerusalem and Judea before beginning his journey around the Mediterranean. His first letter was addressed to the Galatians, and the records of his writings gave the beginnings of the Christian New Testament canon.