Matthias called as a special witness of Jesus Christ

Following the fall of Judas from the apostleship, Peter brought the
remaining apostles together to choose a new apostle. Acts 1, verse 26
says the lot fell upon Mathias.

Acts 1, verse 22 describes one of the principal functions of apostles,
to be witnesses of the divine mission of Jesus Christ. One of the
evidences of that is his resurrection.

"Beginning from the baptism of John, unto that same day that he was
taken up from us, must one be ordained to be a witness with us of his
resurrection."