After the Resurrection and Ascension of Jesus, the Apostles gathered in Jesus’ name to choose a successor to Judas, who had committed suicide. They determined that it should be someone who had been with them since the time of John the Baptist, and who had been an early member of Jesus’ own ministry. They drew lots to determine who they should choose, trusting in the Holy Spirit to guide their choice.
    
The lot fell to Mathias, whom we know very little about.  He became one of the Twelve and eventually went out into the world to proclaim Jesus and His Gospel. At some point he was martyred, offering the most perfect witness and praise to Christ.