Peter's decision to complete the apostolic group by choosing Matthias (Acts 1.15-26) is probably not a matter of mere organizational concern. Several theological and symbolic reasons explain this step.
1- Restoring the Symbolic Number of the Twelve Tribes of Israel
The number twelve carries strong symbolic value in the Bible. It recalls the twelve sons of Jacob and the twelve tribes of Israel (Genesis 35.22-26). Jesus himself had chosen twelve apostles to represent the renewed people of God (Matthew 10.1-4).
During the last meal, Jesus had promised the apostles:
"You who have followed me, in the renewal of all things, will sit on twelve thrones, judging the twelve tribes of Israel" (Matthew 19.28).
After Judas's disappearance, only eleven apostles remain. Peter therefore seems to consider it necessary to restore the apostolic college to its full strength before the official birth of the Church at Pentecost (Acts 2.1-4).
2- Replacing Judas in Accordance with the Scriptures
Peter does not present this initiative as a personal idea. He states that it fulfills the Scriptures:
"Let another take his office" (Acts 1.20).
He then cites two psalms:
- (Psalms 69.25) ;
- (Psalms 109.8).
In his understanding, Judas's betrayal and disappearance had been foretold, and it was fitting that another take over his apostolic office.
3- Preparing the Witness to the Resurrection
The apostles' principal mission is to be witnesses of the resurrection of the Messiah Jesus:
"So one of the men who have accompanied us during all the time that the Lord Jesus went in and out among us…, one of these must become with us a witness to his resurrection" (Acts 1.21-22).
The replacement therefore had to have accompanied Jesus from the ministry of John the Baptist to the Ascension and to have witnessed the appearances of the Risen One. This requirement explains why only Joseph Barsabbas and Matthias were proposed (Acts 1.23).
4- Waiting for Pentecost with the Apostolic College Complete
The appointment of Matthias takes place before the coming of the Holy Spirit (Acts 1.26). Luke seems to show that the nascent Church enters a new stage with the twelve apostles together. On the day of Pentecost, it is "the eleven," once again become "the twelve," who together receive the promise of the Father (Acts 2.1-4).
5- A Particular Function, Not a General Apostolic Succession
The text does not show that every apostle had to be replaced after his death. James, son of Zebedee, would be executed later (Acts 12.1-2) and no successor would be appointed. Judas's situation therefore appears exceptional, because he had abandoned his office even before the official beginning of the apostolic mission.
Conclusion
Peter probably seeks to restore the group of the Twelve because he considers that this number carries a symbolic meaning tied to the new Israel announced by Jesus (Luke 22.29-30). Judas's disappearance was not to leave this mission unfinished. By choosing Matthias in accordance with the Scriptures (Acts 1.20), the disciples prepare to receive the Holy Spirit and to become the official witnesses of the resurrection of the Messiah Jesus to the whole world (Acts 1.8).
From a historical point of view, this appointment is remarkable, since Matthias does not appear again in the book of Acts. Luke's main interest is therefore not to recount his career, but to show that, at the moment when the history of the Church begins, the apostolic college has regained its symbolic fullness.