Peter stood up in the midst of the brothers (there was a group of about one hundred and twenty persons in the one place). He said, "My brothers, the scripture had to be fulfilled which the holy Spirit spoke beforehand through the mouth of David, concerning Judas, who was the guide for those who arrested Jesus. He was numbered among us and was allotted a share in this ministry. For it is written in the Book of Psalms: 'Let his encampment become desolate, and may no one dwell in it.' And: 'May another take his office.' For it is written in the Book of Psalms: 'Let his encampment become desolate, and may no one dwell in it.' And: 'May another take his office.' Therefore, it is necessary that one of the men who accompanied us the whole time the Lord Jesus came and went among us, beginning from the baptism of John until the day on which he was taken up from us, become with us a witness to his resurrection." So they proposed two, Joseph called Barsabbas, who was also known as Justus, and Matthias. Then they prayed, "You, Lord, who know the hearts of all, show which one of these two you have chosen to take the place in this apostolic ministry from which Judas turned away to go to his own place." Then they gave lots to them, and the lot fell upon Matthias, and he was counted with the eleven apostles.

Our Lord Jesus Christ himself declared what he was, what he had been, how he was carrying out his Father’s will, what obligations he demanded of men. This he did during his earthly life, either publicly to the crowds or privately to his disciples. Twelve of these he picked out to be his special companions, appointed to teach the nations. One of them fell from his place. The remaining eleven were commanded by Christ, as he was leaving the earth to return to the Father after his resurrection, to go and teach the nations and to baptise them into the Father, the Son and the Holy Spirit (Mt 28:19). The apostles cast lots and added Matthias to their number, in place of Judas, as the twelfth apostle. The authority for this action is to be found in a prophetic psalm of David. After receiving the power of the Holy Spirit which had been promised to them, so that they could work miracles and proclaim the truth, they first bore witness to their faith in Jesus Christ and established churches throughout Judea. They then went out into the whole world and proclaimed to the nations the same doctrinal faith. They set up churches in every city. Other churches received from them a living transplant of faith and the seed of doctrine (…) They bear witness to their unity by the peace in which they all live, the brotherhood which is their name, the fellowship to which they are pledged. The principle on which these associations are based is common tradition by which they share the same sacramental bond. The only way in which we can prove what the apostles taught – that is to say, what Christ revealed to them – is through those same churches. They were founded by the apostles themselves, who first preached to them by what is called the living voice and later by means of letters.