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.

The four canonical gospels

There are four canonical gospels that have been ascribed to various individuals.  The Gospel of Matthew, from around 70-100 CE, was attributed to Matthew, the apostle.  The Gospel of Mark, from around 60-70 CE, was attributed to a companion of Peter called Mark.  The Gospel of Luke, from around 80-90 CE, was considered to be a traveling companion of Paul.  The Gospel of John, from the later 90-100 CE, was attributed to the apostle of Jesus named John.