The two-source theory

Dating an ancient document is never an exact science.  However, today general scholarship about the New Testament books holds that the short Gospel of Mark was the oldest Gospel.  The first letter of Paul to the Thessalonians was the oldest document, from around the year 50 CE.  Mark, with a hypothetical other source (Q=Quelle) that is now lost, became the source for the Gospels of Matthew and Luke.  The Gospel of John was generally considered to be the last of the gospels, around 90 -100 CE.

Marcion canon (140 CE)

One of the earliest attempts at solidifying a Christian canon or list of books was made by Marcion of Sinope (85-160 CE).  He rejected the Hebrew Scriptures, so that other Christian leaders denounced him.  Thus, he was excommunicated from the proto-orthodox Christian Church community.  However, he was the first to publish his own list of New Testament books around the year 140 CE, that included 10 letters of Paul and the Gospel of Luke.