According to the New Testament, Matthias was chosen to replace Judas Iscariot among the apostles.
The account appears in Acts 1. After Judas’s death, Peter said that the replacement should be someone who had accompanied Jesus’s followers from the time of John the Baptist through the resurrection. Two candidates were put forward: Joseph Barsabbas, also called Justus, and Matthias.
The gathered believers prayed for guidance and cast lots, after which Matthias was counted with the eleven apostles. This episode takes place before Pentecost, when the apostles are described as receiving the Holy Spirit. A common mistake is naming Paul, but Paul’s calling and apostleship occur later and are presented separately from this replacement process.
The New Testament does not provide many further details about Matthias’s later life. Later Christian traditions give differing accounts of his mission and death, so those traditions should not be confused with the short canonical account in Acts.