In the New Testament
Matthew is identified in his own Gospel as a tax collector (telones) working at Capernaum when Jesus called him with the simple command "Follow me" (Matthew 9:9) — Mark and Luke's parallel accounts (Mark 2:14; Luke 5:27) call the same figure "Levi," son of Alphaeus, leading most interpreters to conclude Matthew and Levi are the same man under two names, a common practice at the time (compare Simon/Peter, Saul/Paul). Tax collectors in first-century Judea occupied one of the most socially despised professions available — Jewish men working, in effect, as agents of Roman occupation, extracting money from their own people and widely assumed (often fairly) to skim off the top. Matthew's calling is immediately followed by a dinner at his house attended by "many tax collectors and sinners," provoking the Pharisees' famous objection and Jesus's reply that "it is not the healthy who need a doctor, but the sick" (Matthew 9:10–13) — making Matthew's own calling a live illustration of the Gospel's central claim about who Jesus came for.
The Gospel bearing his name
Early church tradition (again, Irenaeus and Papias before him are key early witnesses) attributes the first canonical Gospel to Matthew, written originally, some traditions claim, in Hebrew or Aramaic before a Greek version circulated — a claim modern scholarship largely doubts in its strong form, since the Greek text we have shows clear literary dependence on Mark's Gospel (most scholars hold Mark was written first and both Matthew and Luke drew on it), which would be an odd choice for an eyewitness apostle needing only his own testimony. Whatever the precise compositional history, the Gospel traditionally bearing Matthew's name is distinctively shaped for a Jewish-Christian audience — dense with fulfilled-prophecy citations ("this was to fulfill what was spoken by the prophet"), structured around five major discourses echoing the five books of Moses, and deeply concerned with Jesus as the promised Messianic King and true interpreter of Torah.
Later mission and martyrdom tradition
Ancient tradition is notably inconsistent about where Matthew ministered after Jerusalem — Ethiopia, Persia, Parthia, and Pontus all appear in various early sources, with Ethiopia the most commonly cited destination in later tradition, giving Matthew a claimed role in some of the earliest African Christian tradition, alongside the more historically substantiated accounts of the Ethiopian church's founding. The manner of his death is similarly disputed even within the tradition itself — martyrdom by sword, spear, stoning, or burning are all variously claimed, and the Roman Martyrology itself, per the Catholic Church's own reference sources, acknowledges the manner of his death as uncertain even while affirming the fact of his martyrdom.
Why it matters
Matthew's story — a man in one of the most compromised, collaborationist professions available to a first-century Jew, called with two words and immediately at the center of controversy over who deserves a place at the table — is one of the New Testament's clearest pictures of grace extended specifically to someone with no claim to deserve it, which may be part of why the tradition eventually placed his authorial voice behind the Gospel most concerned with fulfilled promise and rightful kingship: the very last person anyone would expect to write convincingly about who really belongs in the kingdom.