Matthew’s body ended up in Salerno according to a medieval translation tradition: after his death, his relics were said to have been moved around several times before being brought to Salerno in 954 by the Lombard ruler Gisulf I, and then placed in the cathedral crypt after they were rediscovered and enshrined in the 11th century.

The story in order

  1. Tradition says Matthew died as a martyr, though the exact place and details are uncertain.
  1. His remains were later said to have been taken west, first to Brittany and then to Velia in southern Italy.
  1. In 954, the relics were transferred to Salerno on the orders of Gisulf I, according to medieval Salernitan sources.
  1. In 1080–1081, they were rediscovered and placed in the new cathedral crypt, where they are venerated today.

Why people believe it

The Salerno tradition comes mainly from medieval chronicles, especially the Chronicon Salernitanum , rather than from the New Testament itself, which says almost nothing about Matthew’s later life.

So the Salerno tomb is best understood as a long-standing relic tradition, not something that can be verified with modern historical certainty.

Short version

Matthew’s body is said to have reached Salerno through a chain of relic transfers, with the decisive move happening in 954 and the final entombment in the cathedral crypt in the 11th century.

TL;DR: medieval tradition says Matthew’s relics were moved from earlier locations in the Mediterranean world to Salerno, then enshrined in the cathedral crypt after rediscovery.