In those days Peter stood up among the brethren (the company of persons was in all about a hundred and twenty), and said, [16] "Brethren, the scripture had to be fulfilled, which the Holy Spirit spoke beforehand by the mouth of David, concerning Judas who was guide to those who arrested Jesus. [17] For he was numbered among us, and was allotted his share in this ministry. [18] (Now this man bought a field with the reward of his wickedness; and falling headlong he burst open in the middle and all his bowels gushed out. [19] And it became known to all the inhabitants of Jerusalem, so that the field was called in their language Akel'dama, that is, Field of Blood.) [20] For it is written in the book of Psalms, `Let his habitation become desolate,and let there be no one to live in it';and `His office let another take.' [21] So one of the men who have accompanied us during all the time that the Lord Jesus went in and out among us, [22] beginning from the baptism of John until the day when he was taken up from us -- one of these men must become with us a witness to his resurrection." [23] And they put forward two, Joseph called Barsab'bas, who was surnamed Justus, and Matthi'as. [24] And they prayed and said, "Lord, who knowest the hearts of all men, show which one of these two thou hast chosen [25] to take the place in this ministry and apostleship from which Judas turned aside, to go to his own place." [26] And they cast lots for them, and the lot fell on Matthi'as; and he was enrolled with the eleven apostles.
Read Pope Benedict XVI's teaching on the Apostle Mathias in The Apostles